I've heard of them scraping algae off surfaces but they are not going to be as good as a couple small plecos at keeping tank walls clean of surface algae. And it will depend on how hungry you keep them. Small tilapia have a powerful protein hunger. Adult tilapia can actually filter feed on free floating algae.
Thanks TC, I just want to take advantage of the algae and phyto-plankton in my pool. I'm not real interested in keeping the sides particularly clean. I have fat-head minnows, but I'm looking for something that I eat later.
If you feed the tilapia enough to get them to grow fairly fast so you can eat them later, they probably won't be all that interested in scraping the walls of the tank but tilapia can filter feed once they grow up so they will eat floating aglae and phyto-plankton.
I have mossambicus tilapia, and they graze like crazy, even when well fed. I think the algae habit may vary by species, but definitely mossies do at least help control algae.