i have had acouple emeralds in the past. i found them to be very agressive when they grow larger, also like rescuediver stated, knocking everything over. they are very strong little crabs, even when they are very small. didnt really ever notice them doing a good job on cleaning the algae. HTH
I have a decent size emerald (1.5 inches across his body) and he eats snails and algae, but my fish are all so territorial he stays away from them. They're always chasing him away. It's pretty funny watching him run from a blue damsel that's half his size. He does that "hey, hey" thing like the crabs in "finding nemo" but hasn't grabbed one yet. He's pretty slow and the fish are pretty fast.