That's mystic syrup, you get it from tapping a mystic tree.
You get the mystic tree seed recipe (which you can plant like an acorn, pine cone, etc. to plant the tree) from the foraging mastery. The recipe takes 5 of each regular tree seed (acorn, pine, mahogany, maple), the tree itself takes forever to grow, the same length of time as a mahogany but affected even less by tree fertilizer