Yep, just tannins and it's all completely normal. It will stain the water but never the tank. You can remove the tannins with water changes or just leave them. I personally looooove the look of tea-stained tanks but some people hate them, it's all up to you.
No reason to worry. You're betta will enjoy it more than clear water in my opinion (mine do). It's all personal preference though. In my one 8g I have 2 pieces of mopani and 2 rooibos tea bags to get it as dark as possible