Goop Guy is character in High On Life. He's a small purple alien with a yellow wide cape in Port Terrene. He is selling Goop, a Furgle excretion, that makes his customers blend in with G3.
There are other "salesmen" in other planets who are called the "Goop Bros", Stan is in Blim City selling alien cum and another in Zephyr Paradise or Port Terrene selling shit.