This is the second one of these critters I've had. I don't think they're particularly harmful, they mostly eat mulch, but they're unattractive and I'm not too sure I want my mint covered with slime mold goo.
Still, they're interesting creatures. I'm a bit of a fan of them, after watching a documentary on slime molds some 10 years ago (I was in high school, I think, so more like 15 years ago now, sigh).
Since this beast is more or less unkillable, I'm sure he'll re-appear somewhere else the next time I garden...I'd been joking occasionally about wanting a pet slime mould, but not this particular species, it's unusually disgusting :)