Was trying to figure out what was on my niger. On closer inspection. Looks like he got nailed by a bristle worm. Lots of bristles sticking out of him, on his side.
Sounds odd unless it went and rubbed against one while burrowing? Maybe try for a close look to see if the bristles aren't the ends of parasitic Copepods, some species have tubular structures that stick out of the host. You should see a bump or other damage at the entry/exit point.
I know how he feels, I used to have a tank and I can swear I was breeding Bristle Worms, Every time I'd put my hand in the tank, I'd get bitten big time, my hand was swollen and they hurt.