New snail in the aquarium?

Hi, I have three apple snails. I’ve had them for almost 5 months or so. The aquarium has not gotten new greenery since the snails came, new sand, rocks or wood.Today I suddenly saw something that looks like a different kind of snail in the tank. Is it a snail? If so how could it get there? Is it something else? should I remove it? Thanks for any input.