I've read a couple of authors who considered science-fictional slime girls; one had them be, effectively, macro-scale amoebas with fine control over their cellular wall, and another had them be a hive-mind of colony creatures, where the individual microscopic creatures weren't sapient and would be shed quite frequently — much like a real slime mold but with more cohesion. Neither would probably pass muster as real evolved creatures, but we are, after all, talking about fiction.