Eminent domain is a process where the private property is seized (with the government required to pay fair market value) in order to use that land for some public purpose that it is not currently being used for. This could potentially be done to property owned by Elon Musk, but not for the justification that he has too much power. And also it must be specifically land that is seized, not equity in a company.
If congress passed a law seizing his property, or the government initiated eminent domain against all of his company's properties, it would be obvious that it was Elon Musk being targeted and not any legitimate public need for a piece of land. This is a called a bill of attainder and is unconstitutional. https://en.wikipedia.org/wiki/Bill_of_attainder
Oh, and you may want to look up eminent domain. The government regularly seizes private property and even does so effectively on behalf of other private entities. If you’re actually consistent on your framing then you’d have to admit you’ve lived your whole life under what you consider a fascist government regardless of political affiliation. For example, here’s [2] Trump using the government’s eminent domain power (before he was president) to take property from another private entity. And here he is doing the same thing [3] as president.
[1] https://en.wikipedia.org/wiki/State-owned_enterprises_of_the...
[2] https://www.cato.org/commentary/donald-trumps-eminent-domain...
[3] https://brooklynworks.brooklaw.edu/blr/vol84/iss2/3/