Well, the snails and hermits and other CUC won't be able to take care of everything. And if you have more detritus building up, and not enough snails/hermits then that's when you will begin to see the buildup.
You can either acquire some more snails/hermits or other CUC, but i would strongly advise against getting a cucumber. These usually die in our aquariums because their food source runs out very quickly. Also, most of the cukes' for our aquariums eat algae, and not detritus or waste like hermits and snails do. Plus, if anything, a cuke would need a very large and completely established tank where an algae source will always be available, in order for it to have any remote chance of surviving.
Another way to clean all the detritus build up.. is to do it manually. During a water change, you can use a python hose, or any hose or gravel vacuum to siphon out as much waste/detritus as you can.
Also, the red stuff growing on your sand, is it thick and slimy? If so, it is most likely cyanobacteria. Pictures of this stuff would definitely help us out more to identify what it is exactly.
You should read this article to find out if it is infact cyano
, and if it is, how to deal with it and get rid of it completely: