What else do you have in the tank? It had to have been introduced from somewhere. It's highly doubtful that it came in with the algae eater, but not impossible. Sometimes snails are swept into the net when netting fish and eventually end up in the home tank. When you introduced your algae eater, did you pour all of the fish bag contents into your tank, i.e., water and fish?
What kind of algae eater did you get? Some are aggressive, or end up being aggressive, and shouldn't be housed with slow moving fish.
David