Slime moulds are non-attackable clusters spawned by Croesus when battling against it. It will spawn within a short vicinity of a player, and chase them at walking speed for a few seconds. If it is able to make contact with a player (regardless if it was their intended target), it will dissolve and reduce 25% of the player's maximum skilling stats. If it is unable to reach a player within a certain time limit, it will dissolve on its own without damaging anyone's stats.
If players are struck by two of them in a short period of time, they may take enough skill damage to be removed from the Croesus encounter, as their damage is the highest single hit Croesus can utilise.