Well one thing to try id you are not useing option 43 or DNS to help the LAP join the WLC is to place the AP in the VLAN 30 subnet and let the ap join. Then tag the port for vlan 50 and reboot the LAP. That should work for you unless the vlan from the switch the ap is connected to isn't forwarding to the core.
I have joined the VLAN 50 AP to controller with this command and have connect the wireless client, but the wireless client of VLAN 50 (static IP's)not ping the gateway address i.e. 10.0.50.1, what should I do now ? whereas the wireless clients of VLAN 30 (DHCP based IP's)are working perfectly.