What might the tradesperson have done to my garage floor.

Hi I would ask the contractor if he has any of the original floor material to colour match the second coated areas.

Was the slurry used to create a dry floor ? If yes it’s possible the damp in the floor is showing and making the slurry darker Unfortunately you could have various reasons slurry failed ,not installed properly,no anti sulphate used ,etc etc Test the area with a damp metre if the dark patches give a high damp reading it’s failed

I'm not really sure why he was applying the slurry, but if your not happy with the results then you should approach and discuss your issue with the tradesman and most of us will do our best to rectify with minimal cost if any charge at all. One would hope that it's easily put right. Or ask him to apply a floor finish to give a better finish, there are many options of finish available.

Sounds like a bodger and blagger. Get an actual builder to come have a look to determine how bad the problem is, to provide a quotation to rectify your floor. Then get in touch with bodger and tell him you want him to fix the floor in the exact method the real builder has quoted for or you want the money off him to pay the real builder to do the work. If he's uncooperative then take the issue to trading standards. They will want to see the bad work as it is. So until you have either the bodger fix it or pay for the real builder or trading standards survey it, you'll be living with the dark patches. If you have to get trading standards involved then once they have surveyed, you'll have to pay a real builder to do the work out your own pocket, then in about a year or longer once the case has been Infront of the magistrates you'll be able to claim the money back in compensation from bodger via the court. Its a Really long process and I'm sorry to hear you've had subsidiary work completed.