Constitution
Michael figured that it wouldn't be easy to hide the marks around his eyes. Even without activating Spirit Eyes one could see them, though subtly.
But then again, Michael didn't really intend to explain anything, in the first place. He felt like it would be the best idea to let others make their own assumptions about the apparent changes of his body – just like Zeke did. It bothered Michael a little bit but it was something that would have happened sooner or later. He couldn't hide his secrets forever. The only thing of importance was to hide it until he was barely strong enough to protect his skin from High Society and other greedy bastards.
'Well, whatever.'
In the worst case, Michael could blame the Tekur for the changes of his body by saying that he procured some tome and that he started practicing after he comprehended something. It was really not that troublesome. Probably.