With the limited facts to go on, I'd definitely recommend you check your ammonia levels in the tank, and do some good volume water changes. Provide the pleco with room to hide, where the cichlids can't nip his fins also. Hope that helps.
It could be the nitrate or ammonia thing, I did use the pleco to cycle the tank. But he did not show these signs until 2 weeks after i put him in there. The other fish do not show any similiar signs.
So you think your tank cycled in less than two weeks? That's usually when you see the first ammonia spike. Have you tested it? If you have another tank running, you could supplement bacteria from the filter media from it. Good luck.