From what I understand of the controller messages, this one means that the client (mobile xx.xx.xx.xx.xx) mac address was associated to an unknown username (Open auth SSID or probing client), and then to another username. Username unknown still can't be removed as an association to this mac address.
This can be very normal, if the client is a windows box probing around. If not, and uf this problem creates issues in the network, I would try to troubleshoot it with a client a find what triggers the message before TAC case...
