Either its shedding (normal) or melting (bad). With shedding it looks more waxy and will close up for a few days and shed the skin, which is how some of them grow, though they dont always do this.
If its melting, which to me sounds more like what you're doing, then more info is needed. We gotta have everything from ammonia, nitrite, nitrate, phosphate, salt level, lights being used, pH especially, alk, and all that. There's too many reasons why it would do that to know without more info.