Don't know about the black circles but necrosis may be setting in the greay patches. Try and keep perfect water parameters for the next couple of weeks. He probably will heal up. If it starts to spread you might quarantine him and use meds to help the healing process. He may loose an arm in the process but if the water is good and you medicate it should heal and grow back in time.
Message to all aquariasts!!!!!
Anyone who lives within their means suffers from a lack of imagination.