Not a mimic

The mimic was getting hungry and kept eating the local goblins. in return for not getting eaten, the goblins made up a sign showing where there was treasure (Not a Mimic, honest) and laying a trail of that useless gold junk adventurers like to pick up for some reason. The mimic gets easy meals, the goblins get to keep the stuff the adventurers were wearing or carrying and the pests from above are eliminated from the catacombs for a while. It's now a win win for the Mimic and the goblins.