New grout yesterday, efflorescence today - what do I do?
The efflorescence is just a bit of salts that precipitated at the surface of the grout. Stop wire brushing it and wash it with a scrub brush, with a dilute solution of muriatic acid and warm water, that should take care of the problem.

Next time use a bit less water when mixing your grout, if it is soupy then that is too much water. Always let your grout slake, as (likely) described in the mixing instructions.

You should be sponging as you go, dirty sponge/clean sponge, with well wringed sponges, rinsing and changing bucket water frequently. This helps remove salt-carrying water from the grout surface before it can dry into an efflorescent crust or residue. Always follow up with an acidic "haze" sponging.