If foam is being produced there, that means your skimmer is not pulling it out. Just scoop it out, it is waste. A little is ok. Enough to notice could mean your skimmer is under performing. Or it could be that your overflow is producing super fine bubbles and collecting the waste before it gets to your skimmer. Have any sketches/descriptions of your setup? Where is your skimmer in relation to the drain hoses? How large is your display, sump, skimmer rating, return pump rating? You have a filter sock or bubble diffuser method? When did you last clean your skimmer? When did you last clean your skimmer pump and hoses?

This could be cosmetic… or could mean that you have a buildup of waste.