Can anybody tell me what is this?

I bought an RES from an animal fair in my town. Had no idea about turtles, the shop owner said It's maintenance is easy and simple, and me being a curious pet lover had no idea.

After I brought it home, it didn't seem healthy. It has swollen eyes. I did a lot of research on the internet.

Now I'm letting it sunbath everyday and heat lamp in the evening. UV light is on its way and is gonna reach tomorrow. They had stored the turtles in such bad conditiona in the shop.

Today I saw this weird spot on the ventral part of the turtle. I Have no idea what it is, can anybody please help? And suggest me how to get rid of this spot.