This usually means water on the acidic side of neutral, and the blue-green color is copper coming off the pipe itself.

Low pH water keeps dissolving a little of the copper it runs through, and that copper ends up in the basin under the drip.

Look for blue-green streaks under taps and around the shower drain, green marks on a white bath, and pinhole leaks in older copper runs.

Do not buy a cartridge or a softener for this. Neither one raises pH, and the pipe keeps giving up copper while the cartridge gets changed.