Settler's League - Fetid Pool, Invulnerable Monster

Was playing on the Settler's League, clearing out the Fetid Pool. Fully cleared it out until it said one monster left, and I find him standing near the new guys who were happily mining away.
-- Guess the Dripping Dead didn't get the memo that only living creatures are allowed in Kingdsmarch.
-- Sadly, when I went to escort him away, nothing seemed to affect him. So, looks like I'll have to clear out the pool again at some point and hope it doesn't recur.