UGGHH pond snails. I tried several different varieties of non-aquatic plants in one of our tanks over the summer. I suspect the little pests came in off of one of those. I don't think they do anything bad really, they eat dead plant matter. I just find them irritating to look at. They survived a bleach dip when we transferred some of our plants to another tank. The 70% of the dipped plants didn't survive but the snails did.
So now I transfer plants between freely between invaded tanks. Plants from the clean tanks can go to invaded tanks but only plants that have been in quarantine for a month or two go into the clean tanks.