Mud-Like Substance Emanating From new Concrete Slab

[bobbyfluffy]

My builder laid a concrete slab for my extension a couple of days ago. While it has been fairly dry it has been rather cold so I expect curing to be slow. However it rained today rather heavily and the part of the slab near the window opening got soaked. I noticed these small eruptions on the surface around the wet areas that look like they are oozing out some sort of brownish substance that feels like soft mud. Would any one know what this is, and is it normal?

[michaelscad]

has the slab got steel mesh in if so, it's too near to the surface,mesh shouldbe at least 25mm below
if not then maybe some metal like nails were dropped when laying the slab it's rust

[bobbyfluffy]

Hi, thanks for your response. No there is no steel mesh, its just concrete, I can't see anything that looks like nails on the surface but where these stains appeared there appear to be small dark stones..