Are you referring to dinoflagellates? It is thought that they thrive when there are sufficient silicates in your source water as they require this element to grow their tests (shells/skeletons). If you are using an RO/DI to clean your water you can consider this a short term issue, just part of the several waves of different kinds of nuisance algae you'll experience as part of breaking in a new tank. You will probably find that it lifts out in sheets and can be vacuumed up. It usually stops growing within a week or two and then disappears.
I don't know why you would think GAC would cause dinoflgellates to grow. I have not heard of snails being hurt by it, but I suppose there may be some types that are bad for them. I had heard that fish wouldn't touch dinoflagellates but I have seen yellow tangs munching on it sometimes. I can't see dinoflagellates harming corals unless you leave them undisturbed to smother the corals.
Perhaps you are confusing the type of dinoflagellates that grow in our aquariums with the type that can cause "red tides". As far as I know, those types of dinofagellates are more likely to be seen in a colder, temperate water than the tropical temperatures we keep reef tanks at.
Dave.M