Can wifi admin see what I am doing on FaceTime? I don't mean can they see I am using FaceTime, but can they intercept the actual audio or video?
Apple FaceTime uses end-to-end encryption, this means that the call is encrypted so only you and the person you are calling can view/listen to the call.
End-to-end encryption is very common nowadays, it'd be very worrying if anyone could snoop in on your private voice and video calls.
Unless the device belongs to the company, in which case there could be a Remote Administrator program installed, there is no way for anyone to snoop in on your calls unless they're standing behind you.
Here is a quote from an Apple White paper Released February 2014
FaceTime is Apple’s video and audio calling service. Similar to iMessage, FaceTime calls also use the Apple Push Notification Service to establish an initial connection to the user’s registered devices. The audio/video contents of FaceTime calls are protected by end-to-end encryption, so no one but the sender and receiver can access them. Apple cannot decrypt the data.
FaceTime uses Internet Connectivity Establishment (ICE) to establish a peer-to-peer connection between devices. Using Session Initiation Protocol (SIP) messages, the devices verify their identity certificates and establish a shared secret for each session. The nonces supplied by each device are combined to salt keys for each of the media channels, which are streamed via Secure Real Time Protocol (SRTP) using AES-256 encryption.