What are your water parameters?
It could be trying to molt.
Sometimes they are trying to molt and for some reason can not. Like poor water parameters, not getting enough calcium/minerals. Maybe old.
Are they/it a more recent addition? How long have you had them?
Did they come from a hobbyist or lfs or shipped?
What is your pH, KH/GH if you know those.
What temp?
What do you feed them?