Tony,
The white stuff is efflorescence -I dont see any leaf stains although the brown could be. It looks like to rust to me.
I think you should hold off on sealing. While sealers breathe it may slow down the transmission of vapor.
There could be more efflorescence and the installer will blame you-I would hold off servicing that stone until the install was completed and it rains a few more times.
Let the Mason deal with it-I would put on my running shoes and run to a nice predictable interior marble job.