Mulm (detritus) is a naturally occuring event in all aquariums. Excess mulm which you describe may come about from stagnant conditions, decaying organic matter, dead animals, overfeeding, insufficient substrate bacteria, and recent increase in livestock to name a few.
It may attribute to increased Nitrate levels with breakdown further into ammonia. My suggestion would be to remove the accumulation on the surface by siphon as part of water change. No need to clean within the substrate unless you are seeing layers of build up in which case, bacteria supplementing may be appropriate while curbing the feeding schedule and portions a bit.
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