Oscar Rubbing Belly on Sand

My Orbic is rubbing his belly on the substrate. Not flashing or scratching against anything. Just lowering down to the sand and wiggling (for lack of a better term) and stirring up the sand. Is this something I should be worried about, or is it just an Oscar quirk?

Every other fish in the tank is fine, including my Red Oscar and Chocolate Cichlid. He's the tank boss, so he's not being bullied. Everything appears fine, with the exception of the water being cloudy, but that's his doing.

ph 6.4
Ammonia 0
Nitrite 0
Nitrate <5
Haven't tested GH or KH, but the TDS is 4
Temp is 83

Just the belly. There's a Red Oscar and a Chocolate Cichlid with him. All are around 5 inches. There are also some dithers in the tank; Bolivian Rams, L200 pleco, and a couple of cory's that have evaded capture so far.