Hi and welcome to the forum
Does the white patch stick out a fair bit or only a tiny bit?
It could be a bad bacterial infection with fungus on top or a tumour, but tumours don't normally grow that quick so I would say bacterial and fungal.
If you can find a broad spectrum fish medication designed to treat bacterial and fungal infections, then try that asap. Don't use anti-biotics tho. Just use a medication designed for fungal and bacterial infections in fish.
To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons.
Remove carbon from the filter before treating or it will absorb the medication and stop it working.
Wipe the inside of the glass down, do a 75% water change and complete gravel clean. And clean the filter before treating.