The AP's Network -> Monitor -> Multicast Status -> Multicast Processing status parameter indicates the mechanism which is the AP is using to transmit Multicast data to the registered SMs.
Multicast Over Unicast will take the multicast stream entering the AP and make a copy of this stream for each SM subscribing to the multicast stream and then send this to each SM as a separate unicast data stream. This allows the AP to send this multicast data stream to an SM just like any other unicast stream and take advantage of rate adapt and ARQ retry mechanisms to optimize throughput to that SM.
Multicast Using Fixed MCS will send the multicast data as one multicast stream over the air for all the SMs subscribed to the multicast stream to use and typically a very robust MCS is chosen so that all of the SMs will be able to hear all of the multicast data stream. There is not an ARQ retry mechanism in use for multicast data.