Well they are and aren't. Alot of times they are confused for other crabs. There are certain crabs that will eat other things than just algae. Better off getting a sally light foot for algae. The true emerald has a red spot on the inside of their arm so i've heard. Its hard to see. If they don't have the spot then use them at your own risk.
Keep in mind reef safe is just a general term. What type of reef safe? Corals? Inverts? Both? Some fish can be kept in reefs but you can't keep inverts, ie Triggers, some fish can be kept with inverts but not corals, ie Angels so you've got to go deeper than jsut "reef safe". There again certain corals such as sps with angels but softies should be ok. Hell coral banded shrimp are "reef safe" but are known killers of smaller fish too.
So before you ever put anything in your tank you should always understand the risks involved with doing such. Every invert/fish is different.