kelly........it can also be there because of being a new tank, excess nutrients, not enough water changes, changing the lighting schedule to an hour or two off break a day, can interupt its growth but plants will still use the stored energy from the lights to continue to grow. its to help halt growth.
vaygirl, as far as the snails go, look up and under all around the tank rim, it seems snails like to lay eggs there. fish and shrimp more than likely will not be able to get to them. also, you could just scrape them off and throw them away. i wouldnt recommend flushing them.