People on here won’t accept “parameters are normal”, just give us some kind of numbers to work with
Other than that do you feed it, do you have any fish or inverts that may be picking on it, when was your last water change, and if you dose, what and how often?
Please detail out your tank parameters? what kind of test kits are you using?
What nutrient export methods do you use?
What is your water change schedule do you have and volume?
What were your lights set at when you took that picture?
How long has it been closed up?