If he still swims around elsewhere otherwise I'd say nothing to worry about. My humu would do a kind of a forward somersault 180 degrees, then roll back upright and take off in the other direction. They're peculiar fish and will find some odd stuff to entertain themselves. I also noticed mine would do something for a week or two, get bored with it, then find a different way to entertain himself. Will definitely be including a trigger in my 150g fowlr I'm working on getting setup.
If he's just staying back there for a day or two without moving about elsewhere though then I'd be a bit more concerned. If it just something he'll go do every now and then, I wouldn't worry too much about it.