My BN Pleco will do this why stressed it is not a good sign.
Check the water right away. I would do a WC.
Are the spots cloudy, fuzzy, or slimy?
20gal long planted tiger shrimp & RCS, 80gal planted discus community, 100gal SA cichlid delhezi, 7 gal betta, 20gal goldfish grow out.