Sounds like it is dead/dying. A cleaner chrimp will go after a dead/dying clam.
Take it out and give if a whiff...you'll know if it's dead. If it is, take it out of your tank ASAP...you don't want that sucker decomposing.
Now you need to figure out whay it died. First, what size tank do you have and what lights? What are your water parameters and other tank residents?