Tantalosweed is a type of plant growing near The Ambrosia River. When you reach for it, it moves backwards. To grip it you must either be very fast or grab it indirectly. Similar to sponges, they absorb water.
While tantalosweed can't be used in potion, it sponge-like composition makes it great at cleaning up spilled potions. It natural immunity to the effects of potions also prevents the user from the affects of the potion.