[BoomerD]

Hey, for a change, my evil twin got some right...:P

OP, you have a fiddler crab? or just hermit crabs? NOT the same thing...

As for the ich on the damsel, (which it is what it looks like, but could be something else...better pics needed) the only way to eradicate ich is to catch the damsel, put it in a cycled hospital tank (or a new tank with a cycled filter) for 6 weeks. During that time, you reduce the salinity as measured with a refractometer, NOT a swing-arm hydrometer, to 1.009 and keep it there for the 6 weeks. You have to leave the main tank fallow (no fish of any kind) for that period or else the ich can progress in stages of its life cycle and won't go away.
An alternative to the hyposalinity (1.009) is copper. Best to use chelated copper rather than non-chelated, and follow the directions on the box/bottle VERY carefully. Copper is also toxic to fish in the wrong dosage.