It is difficult to tell from the photos, but I think the one fish may have ich, or a protein problem. You had five, when did you get them? Have any new fish or plants been added to this tank recently? I'll deal with the protein issue below.
As for temperature, they are at the lower end of their range, but the lower and higher ends of a temperature range means the fish can tolerate these temperatures but not necessarily permanently. It would be better in the mid-range, 76-77F/24-25C. This in itself is not likely the issue here, but can be a factor.
These fish were likely wild caught, and a better diet would be Fluval Bug Bites, Omega One Shrimp Pellets, and among frozen food daphnia (ideal) and shrimp. Worms can be problematical with Corydoras diet, but these foods mentioned are nutritious and closest to their natural habitat foods which are insect larvae, insects and crustaceans.
The API pellets I cannot find nutritional data for, I am concerned about the protein. The Tetra flakes I would not feed cories; for one thing, protein is 46% which is high. Too much protein in a cory diet can cause white protein lumps, and these may be what I see on the one fish rather than ich. I just cannot tell from the photos. But I would highly recommend you switch foods. Those I mention are not too high in protein, and they have no cereal meal/fish meal fillers.
