It looks like it is some type of polyps. There are many others besides green. I had red in my tank until a fungus eradicated it. I wouldn't put much thought into it though. If it is a healthy snail it will be on the move and most likely the polyps might not ever open up and die off.
On the snails that's neat , it may tear off as it grows and seed the rocks or even the glass , it looks awesome on the rear glass , just be careful with it as it spreads fast and will invade other coral.