The bottom is a Twinspot Hogfish. Wont usually bother corals, but will likely get shrimp, crabs, possibly clams. Normally OK with fish, though can be aggressive with very small species, or stressful to slow moving-slow feeding critters since they swim non stop.
The nudibranch is a cool one, but unfortunatly not a species known to survive in captivity; an obligate sponge eater I believe (Im guessing it is Phyllidia varicosa, or at least a close relative). Be careful; P. varicosa is very toxic, and when stressed out will secrete a poisonous slime that has the potential to kill its tankmates.