sick hermit crabs?

[SammLuke]

hello, I normally have 6 happy hermit crabs, 3 scarlet red and 3 dwarf blue but in the last few days I have noticed I can only see 4 and the ones that are in the tank seem sluggish. We seem to have significantly more algae in the past few weeks as well.
Anyone know what could be happening to the little guys? They never fight, I haven't seen any pieces of them or molted bodies in a while either. All water parameters appear to be on point and all the other invertebrates seem fine. I have noticed what appears to be a ball anemone growing but that's the only change in the tank for about 4 months.

 

[SammLuke]

to add...

I have had them for about 6 months... they are various sizes, we have several different shape/size shells for them. They were moulting and changing shells frequently. They were always running around the rocks and would be very active at feeding times.
They have fought on occasion but never resulted in any damage... usually one throws the other off the rock and then both run off and do something else. the only fish I have seen pick at them is the scooter and he is just trying to steal their shrimp pellet.
I believe they were tank raised but not 100% sure.
tank mates: porcelain crab, blood shrimp, 3 sexy shrimp, 2 clowns, 2 small gobies(blackray, tangaroa) and a scooter. we also have a hammer coral.
Typically they eat algae, shrimp pellets, brine shrimp and cyclopes.
I assume if the missing 2 have died, then they were eaten in entirety by something since i see no parts in the tank.
i have a rena xp3 filter, a bubble hose thing, 30G bowfront tank with 30lbs live rock and 3 inch med-fine sand.