Hi Eric,
Thanks for your answer.
Im not sure its parasite, its might a desease. But its contagious, because i saw other ancistrus in this tank with the same.
The behavior total normal. Eating well etc.
To me it looks like a mutiation on/of it's bristles, like a tumour(or something related to cell division, I don't know if that's the word not native English speaker) of sorts perhaps? Are the plecos siblings?
I saw ancistrus that have bristles that looks like full blown coral fungus and seems to never stop growing and this looks somewhat similiar.
Doubt there is much to do regardless and I don't think it's anything that is, per default, harmful.