I think hermits are omniv.. err (spelling???) they will basicly eat anything small enough to fit in their mouths, which I think includes corraline, my corraline growth always seems to be better with less hermits.
I have 100 hermits in my 90 gallon tank. At least 30 of them are blue legs. Never seen them eating coralline......at least can't tell that they are. If your coralline is growing like it should be and alkalanity/calcium levels are staying constant and in line I don't think you will be able to tell if hermits were eating any. I'd worry more about keeping your parameters in line to spur coralline growth. Eventually your coralline will grow so fast you won't even be worried if a hermit ate some coralline or not. Even if hermits do eat coralline they serve an excellent maintenance purpose in our reef tanks (eating algaes, cleaning up deitrus, cleaning up a fish that dies under your reef, etc.) In the long run your coralline will cover everything. Be patient.........RR
I read an online article (I wish I could find it again) regarding coralline growth. The article described hermits as a major cause of what hobbyists think is coralline erosion.
In the article he expressly mentioned (this is what made it so memorable at the time) that blue-legged hermit crabs were less likely to eat coralline so if you MUST have hermits, the blue-legs were a good choice.
Keeping this in mind, I have read numerous posts about hermits and coralline trying to determine whether this theory was true or not . From personal experiences shared in the posts, I have come to believe that all hermits are capable and perhaps do eat coralline.
As an experiment, I have ordered small microhermits from www.ipsf.com. I will isolate them with one piece of rock to determine if they eat the coralline.
Other than this experiment, I have decided to keep a hermit-free tank, utilizing other non-chemical methods of algae control.