Flashing green on screen

My tv is about 3 years old and we noticed that it began flashing an overlay greenish color on occasion that could best be seen if the movie(picture) was dark(black). Also, once the tv turned off and back on very quickly on its own. We had a tech come in that used some type of device to clean something at the back and the flashing has stopped for now. Would love to know what to do if this problem happens again and what caused it to happen in the first.