yes. ur crab is the one toppling ur corals at night. even small 2" sized crabs can be a pain in the bum! u might want to trap it. it will be too troublesome to tear down ur 8ft tank to catch it..for more ideas on traps, u can search the forum. most of the conventional crab traps are suited for small ones, since urs is so big, u need to improvise and bigger ur trap (:
yes. the tank will need to mature. usually a matured tank is around 8 months to 1 year. I THINK. but yea. need a matured tank. adding too much lviestock too fast will cause an imbalance. sometimes corals seem ok, but over many many months they will start to deteriorate and die. keep a close on on them. as u said, they are doing very well. most likely they really are doing well (: if they are dying, very fast one. few days to a week and all dead liao. unless they are dying a slow death.