Hi,
Thats a really hard one as the pic really doesn't show much as far as details.. it really looks like small pieces of that white substrate..Is that substrate new? Is it possible thats what you are seeing?
If you want to email me a few pics of the fish in question as hi-res, uneditted images, maybe I can bring out some more details. email to
brewmaster15@comcast.net If your camera or phone can take "Raw"images, send those.
short of pics, If I thought it was a parasite,, I would try something like Paraguard, or a salt dip. Corys can be a bit sensitive though so you may want to use doses and concentrations at 1/2 strength initially.
paraguard and corys
https://forum.seachem.com/forum/fres...s-and-rasboras