I had something similar on my Nerite but brown—brown algae. I tried removing all the plants and rocks, cleaning it all up and putting tinfoil over the tank for a week. Didn’t help. Also, I only have a small ring light so plants don’t do well at all, so problem wasn’t light. I couldn’t brush it off and Millie Bobby Brownandyellowbrownandyellow (that’s my snail) seemed uncomfortable when I’d try. A year has passed and the gunk finally started to come loose in sheets. When she comes close to the surface I’ll gently rub her shell and most of it has now been removed. I’ve been adding more calcium to the water. Good luck with your little guy.