I had two of these come in with some live rock. I never worried much about them, but now they've multiplied like rabbits! I have at least a hundred in my 40 gal. They seem to stay on the glass.
I'd say likely a limpet. And, as a word of note... RARELY are things detrimental to the tank. Most of those that are detrimental are predatory in nature.... in order for predatory critters to mulitply, they must have food. So... if you see nothing but a handful of snail-like critters on the glass where you have alot of algae surface, its most likely that they are just eating that algae.
(In the same vein, if you see a bunch of snails on a clam and the clam looks bad and you start seeing more.... )
Dig thru the faq's and you will see most of the animals that are "bad".