Do you mean a sticky but translucent film? This sounds like snails. You may not see them as they sneak out at night to feed on tender tissue like petunias which they love. Use a slug and snail bait and try surrounding tender plants with the bait but do not put it directly on the plants. You can also go out at night and handpick the snails or slugs with a flashlight. A wad of damp newspaper near the plants will become a day time hiding spot for snails and slugs and after a few nights you can collect the damp newspaper along with the snail population. Keep growing, Marianne Binetti