Fey aren't inherently corporeal and the body they manifest in can technically change from one second to the next. Most fey settle over time to a preferred manifestation, if only to avoid having to explain who they are all the time though there are some who will look different depending on who they are addressing.
In this case, the invasion by the spirits has changed the fey to such extend that his manifestation did too.

No, the spirits aren't fey they're roughly embodiment of emotions so strong they become alive. thus you have the Furies, the Muzes and these spirits (which tbh I'm still researching a name for). I'm working on a big flowchart poster thingymado showing the relationships of all species and groups in the CtS universe.