Sue for what? What are your damages? If they accept Medicaid in other cases, there is no misrepresentation. If they reject all Medicaid patients despite that ad, then report them to the state consumer protection agency or AG's office for false advertising, and perhaps also to DHHR, but again you may have no damages and just have to go to another dermatologist. If they take other Medicaid patients and rejected only you, whether yo have a claim for discrimination depends on why they rejected you. If because you are a member of a protected class (race, gender, ethnic minority), then your complaint goes to the state anti-discrimination agency. Being on Medicaid and/or poor, however, is not a protected class and so there is no discrimination. Again, your remedies may be to your consumer protection agency and/or to the Better Business Bureau.
To questioners from West Virginia & New York: Although I am licensed to practice in your state, I practice on a day-to-day basis in Massachusetts. I answer questions in your state in areas of the law in which I practice, and in which I feel comfortable trying to offer you assistance based on my knowledge of specific statutes in your state and/or general principles applicable in all states. It is always best, however, to work with attorneys and court personnel in your own area to deal with specific problems and factual situations.
It MIGHT be false advertising.
You can file a complaint with the state.
Lawsuit? Unlikely. What are your damages?
Get on with your life.
THIS ANSWER IS PURELY FOR ACADEMIC DISCUSSION ONLY AND DOES NOT CONSTITUTE ANY TYPE OF LEGAL ADVICE OR LEGAL REPRESENTATION.