First Emerald Crabs

I just added a 2 emerald crabs to my 25 gallon to try to take care of some algae problems. I've heard that they can be problematic in a reef. What should I be watching out for?

They have been known to go after small/sleeping fish when they get larger

Would they bother my clown in her anemone? They are fairly large.

How large is the clown? I would think that it would be safe, I'd be more concerned with small gobies or damsels, but I'm sure Bifferwine, Little Fish, or Fast will chime in

It's a full size percula. I only have her and a purple stripe dotty back, and it is fast and somewhat aggressive so I'm not worried about it.
Not a threat to any corals though I hope.

Didn't have any issues at first and then caught one attacking my coco worm. It was ripping off pieces of it. Then caught it attacking one of my hermit crabs. We took him back and traded him in. Didn't have any problems with the other ones.

Normally they don't cause problems. But some will go after smaller fish. I have heard of them sometimes taking a liking to corals, but I think that's extremely rare.