[kevin21]

I would do a large water change asap. Doesn't look like you have any coral in there, so a blackout for a few days may help a bit. Manually pull off as much as possible while doing weekly LARGE water changes. You may need to do this for a month or two due to the damage already done. Your rock is either leaching phosphate, you are overfeeding, or you don't do proper maintenance and detritus build up caused severe nitrate issues.

This won't just go away. It needs to be solved via the right methods. No specimen will eat that. It is going to be on you to take care of it.