Should i try an emerald crab!?

I would do the crab over manual removal, specially with bubble algae. I am normally against adding creatures to band-aid problems, but this would be an exception. Manually removing bubble algae could be tough. If you burst one of the bubbles, you're going to have 10 more replace it
The only problems with emeralds is they require quite a bit of algae to sustain them. At this point, my 200 doesn't have enough. This is easy enough to combat though, as I just hand feed my crabs nori about once a week. They go nuts for it and it does them well.
This is all if you can get over the creepiness of the crabs. They are so, so creepy. Like underwater, armored spiders with pinchers. I can hardly stand the looks of mine, and one of mine is huge. Major Ewww.

 

i do have plenty of algae in the tank, just probably none that the crab would eat. If i don't know how big of a green bubble algae i have should i wait, or should i just stick in a thing of nori like you mentioned if i don't have enough bubble algae.

i haven't seen one in person but from the sounds of it they seem really ugly

 

I had both bubble and hair algae, a little of each,added two emeralds a few weeks ago along with some more blueleg hermits and its all gone,all of it.I think the emeralds are cool looking, they move threir claws kind of like little machines operating,shouldn't gross you out if you like kritters.

 

I had an emerald that I really liked. Once I got my nutrients under control, it starved. Be wary of their nutrient needs once you run out of algae, and they should be a great addition.