Strange formation on Hoplosternum thoracatum belly

My beautiful Hoplosternum thoracatum has a strange white formation on its belly. He has it for a very long time (counting a couple of months) and it neither increases nor decreases, it looks like the formation of his own body and not some kind of external object (I may be wrong!). The fish feels well, eats very well, is very curious, cheeky and his behavior has not changed in any way that would indicate any problems. But that thing on his belly worries me very much. What it is, it is dangerous to him and other fish?

The tank is 200L.
All parameters are good. I change about 30% of the water once a week. I feed my fish twice a day, mostly they eat frozen food bought in a specialized aqua store. I use plant fertilizers daily. No angry or aggressive fish, lots of plants and places to hide.

Hoplo cats like most cat fish need fine gravel or sand as they are bottom dwellers. In your pics looks like you have regular gravel which isnt just rougher on their tummies but also traps bacteria between the crevices of gravel. Id assume the white spot is bacteria infection. Im not sure the exact remedy but other posters should know. Couldnt hurt to do a large water change.

need some better pictures showing the fish swimming normally, not weird angles.

Uploaded new pictures. It's best what I have, it's not easy to shoot him in the perfect angle unless I would catch him and hold him in my hand .
The other stuff on him and around him is defrosted frozen food.