A herbalist is someone who grows, sells, or uses herbs, especially to treat illness. In English-speaking countries this often means a person trained in Western herbal medicine: they may run a shop, grow the plants, or sit with you and suggest teas and tinctures. The word is calm and respectful; it is not slang, and it is not an insult. Though actually, students in Taiwan often fold this together with a 中醫師. Both work with plants, but they are not the same job: a 中醫師 here follows Chinese-medicine training and a licence; an English “herbalist” may have a quite different path, or sometimes very little formal training at all. So the English word tells you the person works with herbs. It does not, by itself, tell you they are a doctor.