everyone else seems fine.and none of your other fish are showing the same symptom?
where did all your fish come from? when was the last one added? do you quarantine all new fish?
being that there is one of these 'things' on the body i would lean strongly towards anchor worms.
they are mostly all from a local mom and pop shop
last one was added about 3 months ago
I googled anchorworm pics. This doesn't look the same...