My son ate some mystatin and triamcinolone acetonide cream should I be worried? I mean he seems to be fine.... But I don't know if I should take him to the emergency room or what? What should I do?

Relax. Neither of these is toxic and both come in forms we give a baby by mouth. I would take the opportunity to get the poison control centers number and keep it by the phone. I would also scan the house for other ways baby can get into materials that could be poisonous or dangerous. Babies like to explore and unfortunately, they eat things to see how they taste.
If small amounts, no. Small amounts of these can be ingested with no side effects.