IIRC Barbatos should walk over to the table there to make the free stat potions.
Where is the green dot? Is it stationary or moving around? Did you try interacting in the direction the alchemist should be, based on the mini-map?
I assume you tried leaving the battle tower and returning? Did you try moving between the platforms, or speaking to Sassan?
Do you still have a save before Barbatos disappeared?
If you are in the Flames of Vengeance section of the game (with the Dragon Knight Saga release), there is another alchemist in Aleroth.