Yea, Cuban is a Libertarian-leaning Independent who backed Hillary strongly in 2016. He won't run as a Repub.
I personally think very few people could successfully primary Cruz. But some could definitely damage him. Michael McCaul, U.S. House Chair of Homeland Security Committee, could do some damage. But a long-time GOP figure like him wouldn't run if he didn't think he'd win.
McCaul has a lot of money (prob. could self-fund himself; he's one of the richest people in congress) and a strong resume but he's much older then Cruz and has been in D.C. forever and won't be able to win over young conservative voters he'll need to win.
FTR - I would not back anyone over Cruz. He has my support 100%.
One of Donald Trump's main weapons during the 2016 campaign.