You accidentally became a slime lich! The potion seller has a de-lich potion that will cure you of it.
Wait sorry which potion seller. I want to un-lich but the "Apothecary" in the start town only had the basic growth potions
Just the Apothecarist right in town, she is next to the blacksmith. The potion will pop up once you kill the lich (and/or complete the related quest).
