U+1D54F, MATHEMATICAL DOUBLE-STRUCK CAPITAL X

https://apps.timwhitlock.info/unicode/inspect?s=%F0%9D%95%8F

He took it from a tweet https://x.com/SawyerMerritt/status/1683310992493608960

$ \mathbb{X} $

Musk probably came across the shape as a child and drew it on a napkin to carry it with him, like he did with the Cybertruck design after "beating" Car Designer on either the Apple II or PC