Is the fish rubbing on objects in the tank?
Is it eating well?
How long has the fish been like this?
Has the pale/ white area gotten bigger, if yes, over what time frame?
-------------------------
I was advised to let the tank get dirty to properly cycle it so I’ve only done 1 water change (about 30% 2 weeks ago).
Unfortunately, you were advised badly. Dirty tanks are a breeding ground for bacteria, fungus, viruses and protozoans. Poor water quality and dirty tanks cause most fish health issues and keeping tanks clean with regular water changes and gravel cleaning will prevent most problems from occurring.
The following link has some information about what to do if your fish get sick. It's long and boring but worth knowing. I recommend printing it out and reading it in bed to help fall asleep.
https://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/
-------------------------
If you are cycling a tank with fish in, you should be feeding them a couple of times a week and doing a 75% water change and gravel cleaning the substrate 4-8 hours after feeding. You should also do a 75% water change any day you have an ammonia or nitrite reading above 0, or a nitrate reading above 20ppm.
You continue doing this until the filter has cycled, which normally takes 4-6 weeks. After which time you can feed the fish each day and do a 75% water change and gravel clean the substrate once a week.
New filters should not be cleaned for the first 6 weeks while they are cycling otherwise you can mess up the process and cause it to start again. After the filter has cycled, you should clean it at least once a month and every 2 weeks is better. Filter materials should be washed in a bucket of tank water and re-used.
-------------------------
If the catfish was in the tank while it was cycling, it might have suffered from ammonia burns, which could cause the white areas. This would also allow fungus to get in and affect the damaged tissue.
-------------------------
Do not add any medications yet.
At this stage I would just do a 75% water change and gravel clean the substrate every day for a week and see how the fish looks after that.
If the problem gets worse during that time, post more pictures and updated information and we can look into medications then.