There are so many neat crustaceans in Indonesian reefs that it's almost unfair to lump them all into one page. In anemones and tucked into reef nooks and crannies were a wide variety of shrimps, including the saron shrimp, which is as bizarre of a critter as you are likely to find underwater. It looks like it is wearing a party hat.
Two critters I particularly enjoyed watching were soft coral crabs and mantis shrimp. Soft coral crabs are very well camouflaged in their hosts: they camouflage themselves by eating the polyps (the color makes its way into the crab's body) and by pulling polyp branches over their bodies. Mantis shrimps must be the most anal shrimps in the ocean. We had a lot of fun with one mantis in particular, who was so careful about keeping his burrow clean that we could easily start a pushing war by trying to push little snail shells and bits of coral into his hole. I used to have one as a pet. Also interesting were orangutan crabs, which we found sitting in bubble coral, mushroom coral, frogspawn coral, and hammer coral. Oh, and the funny-looking squat lobsters hiding in barrel sponges were fun to photograph as well.