What is this white, powdery substance growing on my cement?

Geeta is right. Odds are that's efflorescence rather than being a mold or fungi. It's basically a mineral salt that comes through the concrete as a result of moisture. Concrete is porous - it absorbs water like a sponge and it has to go somewhere. You're seeing the results of that on your floor.

You can get rid of it with a power washer in combination with some chemicals designed for cleaning concrete. If you prefer more brawn and less bucks, you can opt for one part bleach with ten parts water, a sturdy push broom, and scrub, scrub, scrub.

Your best way to stop it from coming back is to stop the movement of moisture through the concrete. You will need to seal the concrete from the inside, BUT be careful. You don't want a concrete coating product, because over time they break down and you'll be back to square one. They're just like putting plain paint over the problem. You need a concrete sealer. If you can get it locally, Concrete Treat Sealer works well.

efflorescence