If it is a bunch of small white specks, it could be ick. The fish will get near the return of the filter and hang out. You can get ICK cure from Wal-mart, but you will need to hurry, ick is a killer if not treated fast enough.
Ick is sometimes caused from adding too many fish to an already stable environment. Ick is always in the tank, but if the fish are healthy and keep there slimy coats, the ick can't penatrate the skin. It doesn't seem to affect certain types of fish either. If the mollies have it, but plecos (algea eaters) and bottom feeders don't have it, it is probably ick.