What is this and how do I fix it?

Anyone?

Can you post some more details? How old is the fish? Did you just get it or has it been in this tank for how long? What are your water parameters? Are there any aggressive fish in the tank? etc.

The red area under the white patch is bacterial.
The white stuff is excess mucous covering the injured area.
The fish has a weird shaped belly and could have and internal problem.

What does its poop look like?
Is it eating normally?

Add 2 heaped tablespoons of rock salt for every 20 litres of tank water. Keep salt in there for 2 weeks and see how it goes. If there's no improvement after a few days, post some more pictures. And tell us how long its been like that and how suddenly it came on.