Molts are when crabs shed there "skin" so to speak they do this as they grow. that is most likely where the crab limbs are coming from.
As far as the purple and green algea on sand bed and such, Yes getting the nitrates down is very important but what really cleared it up for me was finally getting good flow in the tank. Putting a power head or 2 down low so it is pushing water 3" or so above the sand bed really helps.
Use a mesh style cat litter or reptile litter scoop to remove the existing cyanobacteria "algea" off the sand bed then get some flow down there.
This and lowering the nitrates show improve your tanks health greatly.