Step 1: Quarantine the fish. Any fish still alive need to be moved into a new tank set up only for them. It doesn't need to be fancy. Just clean water, and nothing you can't stand to risk exposure to. If you notice any fungus growing on the fish, you'll need to treat them with an antifungal treatment.
[Note: Antifungal treatments are very bad for invertebrates so don't keep them in a tank that's being treated with it]
Step 2: Boil the contents. To sanitize the contents of the aquarium, simply boil it. The gravel, decorations, anything that can fit in a pot of boiling water should be done. It's especially important for decor like driftwood, which is the perfect harbor for bacteria and fungus as it's so porous. A few minutes of being boiled should kill the fungus/bacteria attached.
For plastic decorations that might melt in boiling water, just getting them to a simmer should be hot enough. I've heard suggestions of putting them through the rinse cycle in a dishwasher, but I personally don't think it's safe because of the possibility for soap residue. Soap is extremely toxic to fish, and should be avoided at all cost.
Step 3: Clean anything too big to fit in a pot, including the aquarium. Really, if you want to clean an aquarium thoroughly, taking the silicon off, and cleaning the glass, then putting it back together is the most effective way (since silicon is a bit porous and can hold any cleaning solution if not rinsed thoroughly).
Since no one want to really go through that process, there are two solutions. The first would be to scrub everything with a mixture of hot water and antifungal fish medicine. This might not be terribly effective as most antifungal medicines need time to work.
Another solution which is more effective but also more risky, is to use a bit of bleach diluted in water. Scrub everything with the bleach water, then scrub and rinse with dechlorinator, then rinse it thoroughly with plain water.
[Note: If you want to get the glass clear of calcium buildup while you're at it. Scrubbing with vinegar helps to clear the glass.]
Step 4: Re-Cycle the tank. Since now you've basically started from scratch, you'll need to set the tank up like it's brand new. Set it up how you want it, and fill it with new water. I would even wait a few days longer than normal before moving the fish back to that tank, to see if the fungus grows back.
[Note: I'm basing this on the assumption that it is indeed a type of fungus growth in the aquarium. Although it's pretty common to see white growth in gravel after overfeeding, as the old food deteriorates and molds. That's generally kept in the gravel and not around the glass, so I suspect this is something more serious than decomposing leftovers.]