Yeah I also dose biomate and have never seen a mulm build up. Though the stones are to absorb ammonia so I'm not sure how well they work if they have a layer of mulm

Ya, I proved that over 4 years using them. Some people though, like to have lots of different things attached to their tanks. It makes them feel better :) For me, the less I have to do, the better.
But in the end, if you have good rock, stock appropriately and manage your water changes, you shouldn't have any NO3 to worry about.

Eventually if left long enough most of it breaks under its own weight , although not very efficient if left that way. Biomate for sure helps as in your case:)