Erlang can "leak" processes if the process doesn't exit once it's no longer needed. Pony (I assume) will clean up a process automatically once there are no more references to it.
Exactly: it will clean an actor, once there are no messages to it, and it has no work to do, and there are no references to it (or any references come from such actors which have no work themselves -- i.e. are in a cycle).