My first thought when simplicio mentioned trademark was that The Open Group -- which would seem to own the X Window System logo -- might need to try and protect their trademark. If they don't, it's easy to foresee how down the road the similarity could become their problem, not Musk's.
The X.Org Foundation might need to do the same for their X logo even though it's a bit different (and I'm not sure if it's trademarked in the first place).
IANAL, but you do need to defend a trademark in order to maintain it after all.
Of course fighting to protect it would probably be costly and financially risky even if they did have a case.
