Hello, Seul! Great questions. It would sound more natural to say, "I'm at a dermatology clinic (not dermatological)". If you were waiting in a dermatology clinic and said, "I'm seeing a dermatologist", it would be understood that you were seeking medical treatment and not dating a dermatologist.
You could also say the following:
"I'm at the dermatologist's office."
"I'm waiting to see a dermatologist."
"I have an appointment to see the dermatologist."
All of these examples could be used interchangeably and would be perfectly understood. I hope these suggestions help. If you would like to increase your fluency with professional English lessons, I am a teacher on Italki. I am currently accepting new students and would be happy to help!