Looks like velvet. Treat immediately.
Use a formalin product as a dip . And treat in TTM whatever it is you need to get him out of the tank quickly. Velvet does act fast. It can attack the body and if it settles in the gills it usually will kill the fish quickly . I lean toward velvet too . Any new additions to the tank or was the fish itself a new acquisition? . Do you have any copper med on hand?
Yikes .drain the tank and scoop them out with a bucket . I would say a koi net. use a Rubbermaid storage tote from HD or Lows . I would still dip them . If you dont want to use formalin you can dip them using Seachem's Paraguard. It has instructions on how to use a concentrated 1hr bath .