Many different things.
Fish touching it, sand falling on it, crab messing with it, etc.
Mine do that when any of the above things happen.
All the rest appear fine.
I wouldn't worry.
Stuff is like a weed and even if that spot fades, it'll be overgrown in no time.
I had a tank with a million bristleworms once and each time I fed, they'd all come out.
Xenia would shrink fast when these worms went all over them ...