You'd need to keep track of the UID of the Mud that each MudTrail is connected to. And then you'd have:
+System: For each MudTrail
+Mud: Pick instance with UID MudTrail.MudUID
-> MudTrail: Set width to distance(Mud.X, Mud.Y, ...
But this way of leaving a trail will only really work if the player is moving in a straight line.