Could be a variety of reasons, cannot tell for sure. Could be Bacteria infection, cut/abrasion when pushing rocks, bitten by bristleworm, stung by foxgface, etc.
Is it still active and feeding? If it is, can just continue to monitor. If too worried and it’s easy to catch, maybe put it through some medical dip, but you don’t know what type because dunno what’s the cause and then you stress it unnecessary.
Think you monitor first.