In short, API EM Erythromycin has the same active ingredient as Fitiz Maracyn. I also agree with Colu that it is best to set up a separate hospital/quarantine tank.
If the cottony growth is on living flesh and appears as flat, fuzzy/velvety white patches, it could actually be a bacterial infection, such as Columnaris. Columnaris is very contagious and the sick fish should be quarantined ASAP if you think this is what it has. The best treatment for Columnaris is a combination of both Nitrofurazone (Jungle Fungus Clear) & Kanamycin (Kanaplex).
Typically fungus/mold (such as Saprolegnia) tends to grow on dead or dying flesh and will be fuzzy and stick out well beyond the skin. Fungus can be treated with Ich-X.
If you suspect fungus, look at everything in your tank and see if it is growing anywhere else, such as on plants, filter, uneaten food. If you find it in the tank, steps should be taken to remove the items that have fungus on them from the tank and disinfect them with vinegar or hydrogen peroxide (bleach won't kill it).
A cautionary tale, I once had Saprolegnia in my tank and, after siphoning the gravel, a weak fish that had his fin nipped by another fish quickly had Saprolegnia all over his tail where the previously unseen injury was.
Saprolegnia (fungus/mold) infections are often secondary to a primary bacterial infection, so in that case you would want to treat for both fungus and bacterial infection.