Problem with Gold Laser Cory

I bought 4 gold laser cory online and noticed this on one of my corydoras. I am new to aquarium hobby and have no idea what this is. It looks like cracks under the skin. After 2 weeks, I also noticed it on my other corydoras too but not as severe as this one on the same exact location. I searched online but could not find an answer. I have used ichx, api erythromycin, api general cure, and seachem polyguard but does not seem to heal. Does this have to do with temp because I am quarantining them at slightly higher temp. Does anyone know what's the problem with my corydoras?

It will help to post additional pictures, different views, and to describe in detail what you are seeing that believe is abnormal or a problem. By all means, stop adding medicines and treatments unless you have a good diagnosis of the problem. In the single image shown, I don’t see anything wrong. Good luck!

It look like a white spot near the dorsal fin on its back with 4 lines that look like an X. At first I thought it was some kind of parasite attached to the back because when I first acclimate the fish and introduced to quarantine tank, I noticed something that look like a shrimp the size of a grain of rice. On closer inspection, it looked like cracks under the skin of the corydoras. Not sure what caused it and how to fix.

Add some heavy tannins to the tank, after doing a good water change. To me, it seems to be the start of a minor bacterial infection and it's enough that quality tannins may help heal it.