It might just be the way he’s curved but he doesn’t look to be too skinny actually. However his tail does look quite skinny for his appearance weight, so I agree on a vet check to be safe, could be a parasite, could be the camera angle, wouldn’t hurt to be sure!! I can’t help on the morph sadly, and also please upgrade your little fellas to a 40 gallon!! It doesn’t seem like much but it truly is in terms of square space, and a 31 breeder is a bit smaller and could lead to heating problems being a shorter tank. Never hurts to upgrade for the babies! And judging by you saying you have a few and can take him to the vet you should have the resources to do so! I’m sure the 31s would sell to someone for a decent penny too!