I can't really see if it is enabled on the screenshot you posted ?
Also IP Mobility is enabled on the VAP


The reason you don't need IP Mobility is because you only have one controller and your APs are attached to the same controller with Clients using the same VLAN.
One of the reason behinds using IP mobility is in the case you have two AP-Groups or Two controllers with different VLANs in the VAP and users will be roaming between those and you want the IP address to be consistent and in thats when IP mobility will help you
In this link explains it better:
http://www.arubanetworks.com/techdocs/ArubaOS_61/ArubaOS_61_UG/Mobility.php