It's verdigree and it's a sign that the combination of water and air have been in prolonged contact with a copper pipe.
Unfortunately, your pic doesn't show what the pipe leads to but I assume it's a TRV or lockshield. It's possible that water from a minor leak over a long period could have accumulated at the point where the pipe enters the floor. I had a similar accumulation of verdigree on an old lockshield valve to a rad in my current place.
The cause could be any number of things but unless the tiles in the area are loose, I wouldn't worry about a leak in the pipe within the concrete floor.
From what you've said in your post, it could simply be that before the floor was carpetted, regular mopping of the floor has allowed water to seep under the vinyl where it would be difficult to evaporate.
My advice (and I'm not a plumber) is to rub the remaining paint off the pipe because it looks dreadful and use fine grade wire wool on the verdigree. Then just keep an eye on it.
Slightly off topic: why on earth do people paint copper pipes?
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