your tank is new it will go through many changes
this is one of them use a turkey baster blowing off your rocks as you do your water changes
your tank is still full of silicates and it takes time to get them under control so things like this will happen it's a normal phase
you will see brown cayno on sand bed also ,suck it out during water changes try not to remove a lot of your sand bed
this is all a part of a new tank , it all looks nasty but over time it will go away we all have been through these phases your not alone