I've got a lot of ramshorn snails in many of my livefood tubs, some of them grow algae on their shells and some don't, there's one at the moment that has several batches of eggs stuck to it's shell, but there's also a snail in one of my ceriodaphnia cultures whose shell is entirely covered in a massive colony of some sort of colonial rotifer. It looks strange, almost like it has feathers or something. I've seen the same rotifers actually attach and form colonies on the carapace of daphnids before too.