it sounds like he maybe had to forcefully squeeze through in some way, like the opening in the Seal was not big enough even for him alone? He could be harmed by that.
I also thought later last night that maybe it's caused by him simply being outside the Seal - it may be taking effort to be where he's physically/magically not allowed to be.

OHHHH now that you say it, that makes me wonder if his current state outside the Seal is comparable to Ye Zun's energy projection when he partially escapes the Pillar! Ghost Face's skin is even described as "a white that bordered on blue", and Ye Zun is a blue energy projection. :D