If you're not sure what it is, how can we be? I ran a few searches for monkey shrimp, and everything that came up was about brine (sea monkeys), or some band in WA.
Sorry. Get a pic or a description, if you can't get a name.
Pretty reef safe, but don't expect to see much of it. Do NOT mix them in numbers as same sex individuals will fight to the death. (Males have a much longer pair of front arms.) I would personally opt for a more sociable shrimp species, but this one isn't dangerous.
if you want a cool, unusual shrimp look into Sexy Shrimp (Thor amboinensis), also called broken-back shrimp. They retail from $6.00-$10.00 each. I have seen Anemone Shrimp (Periclimenes brevicarpalis) sold as Sexy Shrimp but they are completely different.
Sexy Shrimp are small and boldly colored. They perch on my rock (always in plain view) and 'wag' their tails around. They usually hang around together and always in the same area.
I have read reports that sexy shrimp will eat polyps. I have never observed this behavior in my tank nor in the reef tanks of friends that have them. Mine eat just about everything I give them. They aren't common finds at LSFs in this area, you might have to special order them at your LFS or order them online.
Seen them, and decided they were too small to safely live in our tank. I've heard (my LFS--mostly reliable) that they will eat aiptaisa as well. Not sure, but it would be nice. There was something about them in a recent mag....Hmmmm.....Can't recall. I'll try to look it up tonight and post it.