A button has gone under the agitator. will this harm the pump

Most top loader machines have some sort of fastener in the center at the top of the agitator. You should look right at it when you open the lid. Once you figure out how you remove the fastener, the agitator should pull straight up. It is usually connected by a spline. You may have to pull pretty hard if it has never been apart. Once the agitator pulls out, you should be able to see the debris you are looking for.
I am not familiar with your exact machine, but yours should be similar. Hope this helps.