They are sort of different.
Mulm is the wet product of bacteria, plant and animal waste and rotting material.
Detritus is a collection of things like plant or soil material. For example in the Triops and the Killifish industry, detritus is kept, dried out and added to the eggs to aid hatching.
Technically the words may be interchangeable. But aquarists probably differentiate between the substances. May be better to call detritus the dried product of Mulm?
--Quatermass 09:45, 17 July 2011 (CDT)