Is that Pink Belly Side Neck? It looks like either a shed spot which is not a problem or some sort of fungus which is a problem.
How long have you had the turtle?
What do you feed it?
What size tank does the turtle live in? Is it the only inhabitant? Does it have a basking area? UVB lighting? Does the turtle ever receive direct (not through a window) natural sunlight? What is the basking areas temperature? Water temperature? Type of filtration? All of those are important for us to know in order to help.
I have rescued aquatic and semi-aquatic turtles for many years. I've had my Red Ear Slider for 25 years now and have learned all sorts of lessons. Just let us know the above information and we can try to help.