Could it be a misidentified shrimp? I don't know the fine details to tell them apart, but there is another shrimp that masquarades as a cleaner for protection. Most predatory fish won't eat cleaners since they need thier services, hence it makes a good disguise for a shrimp that doesn't want to get eaten.
Which kind of cleaner do you have? I'm having trouble tracking down the pacific look alike, but some quotes on other mixups:
Caribbean Cleaner Shrimp:
Lysmata wurdemanni (Gibbes 1850), Peppermint Shrimp, Caribbean Cleaner Shrimp.... A reclusive, sometimes misidentified species (there are other shrimp from the area that are similar)...
Camelback Shrimp are primarily red, with white dots and short striped markings covering the body. Their backs are humped up, hence the name Camelback. Often misidentified or misrepresented and sold as the Peppermint Shrimp (Lysmata wurdemanni)
China Sea mixup:
Cleaner shrimp - Periclimenes sarasvati
This species has often been misidentified with Periclimenes tosaensis, a species collected in the South China Sea at 80-90 m depth. Only recently it has been named P. sarasvati.
It's too late for a complete research project, but I do recall that there is a similar problem with the most common tropical pacific cleaner shrimp.
My fish have only went to the cleaner twice. I had originally got him because my yellow tang had black ick, which he cleared. Then I recently added a mandarin without QT'ing first, and I knew something was wrong when my fish started going down to be cleaned by my cleaner. So it may be a blessing in disguise that your cleaner isn't working.
Are your fish even going down to him??
No, this is normal, and it does not mean the fish is about to get something. Cleaners clean parasites, as well as dead skin or things between teeth. But it is important to know that many animals will not maintain wild behaviors in captivity. Some fish may readily take to cleaning (often larger tangs for example) while smaller fish may not.
gcd, what kind of fish do you have?