In my opinion, that is mold, yes. How long was the screed allowed to dry before the foil and laminate were laid?
Ants are a pest. Once you have them... we had them in the old rental apartment every spring and also in between. It was the first occupancy after renovation – the house was affected up to the attic (4th floor). Here in the house now too, after we renovated, but only on the ground floor. They were probably already there each time before. We spray ant poison on the skirting boards (caution with children and pets) and also place bait traps, that keeps them in check. Only the ants themselves know where the creatures come from. Well, we have an old building and you can’t really make it tight anyway. But ants in a new building together with mold and moisture in the floor is also something that makes me listen up.