Never met with this before.
If the sand particles are flat, e.g. flakes or crushed shells, they'd stick far better than granulated sand.
It is possible that the fish is stressed and secrets more slime or stickier slime than usual, albeit I'd doubt that.
You may not be able to do anything except remove the sand or the fish.
Interesting problem. I mean unusual. Keep us updated please, if you will.
It looks like a SA species of the Pseudopimelodidae family. If your water is extremely hard and alkaline and with high TDS (with all that sand and coral etc.), these fish will most likely not be happy in such water anyway. Amazon waters are on the soft and acidic side with low TDS, if any.
The stress from the water and from the sand may be too much for the fish to bear. Beware.