It seems just to be unidentifiable gunk and possibly some hair.
We are careful to use little sieve things in the sink and bath and don't put fat or food waste down either.
Still every month or so I have to go out with a brush and clear the stinky gunk off the grate so water can flow away. The grate looks fine and there is a little plastic hood on it to keep snails and leaves out which to be fair seems to work.
It is pissing me off!
Is there a way to permanently fix it?