I'm a big fan of emerald crabs. I do agree that there's certainly no guarantee that they'll eat bubble algae, but they're quite useful with algae control in general. I'm also a big fan of variety as has been mentioned. Start off with a small CuC and work your way up, especially if you get hermit crabs. They get a bad reputation as snail killers, but they're some of the best scavengers out there, and a hermit crab with a full belly is not very likely to go after snails. Here are a few other types you may consider:
- Brittle or Serpent star: If you have any uneaten food in the tank, they'll find it and eat it. I wouldn't get more than one for a tank your size, though.
- Trochus snails: Very much like astraea snails, but they're able to flip themselves back over if they fall. Usually tough to find locally, but you can find them many places online.
- Nerite snails: No better than any other snail, but I think they look kind of cool, and there are about a thousand different color and pattern variations to their shells. Most stay quite small, so you don't have to worry about them knocking things over.
- Fighting conch: I haven't had much luck with these, and I'm not sure if they just got eaten by my crabs or if my 46g isn't big enough. They're very good at cleaning your substrate, though.
- Sand sifting cucumber: Excellent sand cleaner. You want to stay away from the carnivorous cucumbers, but Tiger Tails and a few other species of sand sifting cucumbers are very well suited for the aquarium.
- Porcelain anemone crabs: OK, not so much of a clean up critter, but they look really cool. Perfectly peaceful, they sift plankton and detritus from the water stream.
- Peppermint or Cleaner shrimp: Actually, peppermint shrimp are a type of cleaner shrimp, but that aside, although I don't believe these guys are effective at doing something like cleaning marine ich off a fish, they're consummate scavengers. The cleaner shrimp especially can get rather aggressive with their scavenging, though, and I eventually got rid of mine, because they were stealing food from my anemone and LPS corals. They also spawn frequently, and although their larvae will never survive to adulthood in a reef tank, it does serve as a plankton source, which may be useful to your corals or possibly other organisms in the tank (no doubt, something's eating them).