Need help fast please !!

It won't be a cleaner shrimp, but rather, a mysid shrimp, of which there are many kinds, but the ones normally found in aquaria are known to look like miniature cleaner shrimp.
You won't just have one in your tank, but if you take a flashlight with a red translucent covering, you should be able to see more a night, swimming along the bottom and between the rocks on the bottom.
Mysids are the salt water version of mysis shrimp.
A goodle search will get you more info and pictures of some of the different kinds of mysids found in nature.
I took some from one of my tanks, (they're hard to catch they're so fast) by dumping off the top of the pails of water change water and locating the ones in the bottom that had been sucked up.
I put them in a 20g tank and feed them brine shrimp nauplii, and now I have hundreds of them, continually multiplying and replacing the ones I sell and the ones I put into my reef tank with the mandarin.