Nice looking pleco. To me, that looks like a fungal infection, although the dark color is unusual. It could also be some type of growth upon a former injury to the barb. I am not certain, just my thought. It should not be contagious to other fish. What I would do it to net the fish out and observe carefully under magnification and to try to move or remove it. Then I would apply a bit of a topical anti fungal/antibacterial; then return to tank and observe for any changes over several days. Good luck!