IMO it could be many things. Some people have had good results using melafix with various things. For myself, I have not yet decided whether it works any better than water changes, salt, temp manipulation, or combinations of those.
For the white patches I would continue to use salt. For how long have you applied salt ? Have you seen improvement ? It could be a bacterial infection, and thus, an antibiotic would be needed. Melafix might help the overall scenario but I don't thing that will cure anything itself.
Good luck, Francisco.