Thanks for posting your query at DoctorSpring.com
I can understand your concern.
From the picture, it looks like accumulation of dirt. Have you tried cleaning it with a wet cloth?
The other possible diagnosis is that you have this after an episode of a wound or a sunburn or an allergy to something that you wore or applied in that area.
I would recommend application of cortisone cream twice daily and a moisturizing cream once daily on the rash.
In case it doesn't start improving it would be best to see your dermatologist who can examine you in person and opine.
Please let me know if you have any queries.