White "poo"???
Anyway, it's probably thermal
adhesive, not thermal paste. Paste doesn't have any adhesive properties and comes off easily. Adhesive is a little more difficult to remove, but it's just a matter of gently scraping it off with a flat-tipped Xacto blade and then cleaning with denatured alcohol or something like that.
Arctic Silver makes two types of adhesives (or epoxies).
Arctic Alumina is the stuff you want to use because it doesn't have the traces of silver that their Silver Adhesive does.