It is normal tea. Lan Wangji only serves regular tea, and it's a brew of tea leaves that actually comes from his home of Cloud Recesses, so it maybe slightly herbal, but he made it sweet with honey and sugar.

"Oh...I caught Wei Ying as my fox. So, I want Wei Ying to be a good fox just for me." He's pink to the tip of his ears. He's not sure how to explain this fantasy other than he likes Wei Ying as a fox with a collar on. "So I want to play with my fox more."