I have a section on a grout line where there's a white/grey dot. Is this a hole? How do I touch it up? Can I just apply grout sealer?
That looks like two things to me (neither of which are of concern): A bit of off-color mixture and a tiny air pocket.
Grout sealer doesn't repair grout. It's a very low-viscosity fluid designed to penetrate the grout and make it watertight. It doesn't leave much of a coating, and it doesn't fill voids. Usually. A heavy application, or several, may fill this. It probably won't mask it very well, though.
If that comically small flaw bothers you, get some matching grout and do a repair. Either just rub some over that area, or scrape out a channel and refill.