I posted about this on r/aquaticsnails, and we determined that this is most likely a stentor colony growing on this little guy's shell. I got myself a macro lens to see it a little better and I wanted to share here too in case anyone who knows more about aquarium microbiology can tell me more about it! Do you agree– is this a stentor colony? Is it possible to narrow down the species (or even a few possible species) without a microscope?
This is a 5 gallon walstad style tank, just over 1 month old. 5 adult ramshorns and an unknown number of baby ramshorns are all that's in there right now.
Posted by mellow_clam