Bleed water
Water that rises to the surface of fresh concrete as heavier solids settle. Normal bleeding is expected; finishing while it is present is one of the classic ways to ruin a slab.
What does bleed water mean?
Bleeding is a natural consequence of gravity acting on a suspension, and a moderate amount actually protects the surface from drying too fast. Working bleed water back into the surface with a float or trowel raises the local water-cement ratio and creates a weak, dusty, easily scaled top layer. Mixes with fine SCMs or high air content bleed less, which shortens the wait but also removes the surface's built-in evaporation protection.
In practice
The rule on flatwork is simple and constantly violated: do not touch the surface while bleed water is standing on it.