I never really see my blue legs eating algae, I could be wrong but they just seem like scavengers to me. But with that being said they are a great part of the clean up crew, I have never had a problem with them. Emerald crabs are great. And they do eat algae in my tank, at a very fast rate too. I had a little patch of hair algae and the crabs just sat there all day eating it until it was gone. But emerald crabs can turn on your tank and be a problem. I had 5 in my 210, but one crab in particular wouldn't leave my maxima clam alone, so the clam would never open up. So I removed the one crab and now the other 4 are great citizens! So if you get emerald crabs keep a close eye on them

