I'm not expert with common plecos, but looks sailfin to me. Not sure if it's ich, could just be dense sediment from aquarium bottom which gets stuck to spiky skin of the pleco. I sometimes see that with my plecos after WC or bigger overall aqarium cleaning.
http://www.planetcatfish.com/common/species.php?species_id=148
http://www.planetcatfish.com/common/species.php?species_id=148
