It looks to be an Angaria tyria or Angaria delphinus snail and here some more information.
If in fact it is, the books I have as well as the information I got from the net, do not let me conclude about its feeding habits, so could be algae eater at the best, but it could be also carnivore or both.
Please take as many photographs covering all angles so that more details for identification are available.
From you description this was a hitch-hiker....perhaps with some coral or something you brought home...well one thing I'm positive about...its not Godzilla ...but keep an eye on it until more accurate information is gathered.