Hi,
The milky marks sound to me like it is an emulsion based sealant, and as such should have been cleaned with an appropriate maintainer.
All you have done is probably used a detergent which is slightly acidic, this will have started to take the top surface of the seal off, but as it was left on the top, wet, it has now become milky.
You will need to get an emulsion seal stripper, completely strip the floor and re seal.
It isnt as bad as it sounds, the stripping will only take a couple of hours, then leave the floor to dry for a day or so and go back and re seal. Don't let the customer see that you feel anxious, they will worry too much and think that there is more wrong than really is: take control of the situation, act as though you've seen it before and are not worried by it and are confident enough to rectify it. This may take you more time and cost you more money....but the client will use you again and again.
Carolann