Sounds like efflorescence, which is dissolved minerals carried to the surface of the concrete by water. It can be removed with muriatic acid, or with a specialized cleaner.

I haven't used this cleaner, but the page gives some good info on efflorescence and other possible causes of your white patches.

http://www.radonseal.com/cleaners/RS-eff...

I have white patches on my imprinted concrete and i have not had it resealed or 4 years as my husband was ill and has since died .what do i do to put it right so that i can have it resealed.