Emerald crabs are hit or miss. Some will attack zoos, others will leave it alone. No way to know until you get one and once they are in your rockwork, you will have a heck of a time getting them out!
Your best bet is to control the source rather than remove the problem. Frequent water changes, nutrient control and eliminating phosphates and silicates are the key. Do that and the algae will go away on it's own.
Hope this helps