After a thorough shocking/vaccuming the algae keeps returning within a few days. I notice after backwashing, when I put the filter back on, particles flow through the return back into the pool. What's going on?

a flocking agent will do the job
look for products with brand names such as pool clear , pond clear etc
the chemical attracts the floating particles or mud/ algae takes them to the pool bottom to be sucked up with the cleaner or taken to the filter
available from most pool supplies shops

Take a sample to a pool shop and find out. If the pool has been green you may have dead algae floating around, a sand filter will never take that out. You need to flock it or new water. Then there is the toy you call a sand filter, OK when everything is perfect but small filters and pumps just won't handle a very dirty pool.

Have you backwashed the unit? That would clean out remaining algae.

Always vacume on waste when you have algea shock at night always

I think Brandon's answer deserves a little more time and a slight explanation... as I think it is Spot On!
Algae dust is usually "Already DEAD" algae!
And normally it is way to Small & Fine to be trapped by the filter.
We here on this forum do not know what type of filter you have, But if you do have a "MultiPort" valve as Brandon suggested you turn to the "Waste" mode, you will not use the filter mode to vacuum, as this will only BLOW the Fine Silt like substance right back into your pool, spreading out into the water to be suspended turning your water cloudy if not green, until it settles, and once again you have a covering of Dead Algae Dust on the floor of your pool. But when the filter valve is in Waste mode, you will bypass the filter and the return line altogether and your vacuumed algae silt will be sucked up into the valve, and OUT the valve to waste. And just like that all of the algae dust is vacuumed out and did not Blow back into your pool.
I just had a situation where we cleaned up such a pool where the customer couldn't get the algae out, we came out and vacuumed to waste. And I by chance took a picture of the partially vacuumed pool and Dust remaining.

If you don't have a multipart valve, then if the pool has no debris but only dust, then and ONLY then, should you use Backwash mode. You NEVER want to vacuum a pool with debris in Backwash mode! NEVER!!! Because the larger debris you are vacuuming will get caught in the internals of your lateral drain assembly or your DE Grids. then it will be very difficult to un clog them, which in turn will reduce proper water filtration efficiency! But the algae dust will be able to pass right on thru. If you have neither of these types of filters because you have a Cartridge style filer, the elements may be able to trap the dust, if it is still to fine to filter, you obtain an additional pool pump, place it pool side, and in the pump out the pump will be two vacuum hoses and away you go.

Good Luck

dead algae is made up of microscopic particles. Most sand filters have trouble filtering out particles that small. I would recommend using a flocculent that will force the particles to settle to the bottom, you can then set the multiport valve to the "waste" setting and vacuum as much of the debris as possible. You can use an additive like fiberclear to help the filter clear the water up quicker.

one of the internal pipes broke or rotted away put in bypass mode and unbolt top slide out unit head repair or replace damage pipe

I too have this problem after treatment for algae, especially if I got a day or two without running the pump, it starts to get mildly green. After adding the algaecide, it kills the algae, but leaves what I call the white dead floating...it takes several days for this to phase out.

If you aren't already doing so, put 3" tablets in the pump basket housing, that helps dramatically. Put a pair of knee-high pantyhose (one inside the other), on the return inside the pool. (you will be amazed what it catches after a few days). Also you can try some "clarifier" to help clear up the cloudiness.

I'm no pool expert, but have dealt with the cloudiness myself. Through trial and error, have learned that cloudy water doesn't necessary mean dirty water. It will clear within a day or two if you are keeping stabilizer tablets in pump basket and using shock once a week. And run the pump for about 8-12 hours a day, it will clear up quicker. And to me, using the pantyhose is an extra-filter and it catches the 'gunk' that the sand in the filter isn't catching. (it can't possibly get everything). Be sure to backwash your pump frequently too.

Hope this helps at least some. Good luck!

DE filter? If so then you need to give the filter an acid bath to purge the dead alge from the filter (Fingers, Grids or Discs). Go with 1part muriatic acid to 3 part water. Let soak overnight. The water should turn almost neon green.Rinse off and reassemble. SHould be Good to go. DE and Cartridge filters should be given an acid bath once a year.

add a clarifier this will make particles bigger and will be more managable either through filter or vac