I'm using my college Wi-Fi on my iphone do they know what i'm doing?

Can they spy?

They can only spy if they think someone is using their network and then they can see what you are doing but I may be wrong but I don't think the computer tells them that someone is using their network.

Network devices have a MAC address, which is similar to a IP address, but permanent and unique for that device. When a end-user device connects with a network, they will use their MAC address to connect. It should be noted that these MAC addresses can be used to identify the purchaser (and usually the user) by contacting the distributor with these ID.

As all data is being sent through a middleman (the wireless connection), any un-encrypted data can be viewed by the middleman, and if they wanted to, falsify some data (similar to man-in-the-middle attacks).So in short, any unencrypted data can be spied on, and with MAC address, the origin can be tracked.

The real question is whether or not your campus does this, and to what extent. In all likelihood, they do track and record MAC address that connect to it for security/legal reasons, but for how long and what else they do is up for debate and will vary from place to place.