It sounds similar to (but possibly different from since I never heard the term "slice universe") the theory of presentism in the philosophy of time.

That's the view that all that exists is the present. What we call the past is what used to be present and what we call the future is what one day will be present, but neither the past or future exist so long as they are not the present.

https://plato.stanford.edu/entries/presentism/