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!
Here’s a better picture they look like kite shaped organisms in a clustered area around the barb. Hasn’t appeared to spread. He had this when he was shipped to me from out of state. Has had them for roughly 8 months now. He had some sort of bacterial disease that tattered his fins and also had holes in his top fin and that cleared up with the MetroPlex treatment. I don’t even know what category these pests fall under so I cannot research the proper criteria for a diagnosis treatment plan.
I don't know what it could be either but it doesn't look good. After 8 months it doesn't seem dire in any way but I agree, get some planetcatfish opinions for what it is & what to do. That last pic is quite good, hopefully they on PC can help.