Some think this is a fungus, while others call it mold. It is neither! ! Fungus is yellowish-whitish in color. This is merely a bacterial infection. It is usually caused from an artificial nail lifting off the nail plate and moisture is trapped between the two. Because there is moisture, warmth and darkness, bacteria grows under those conditions.
It is nothing to be scared of, unless left to continue to grow, then it will become black. The artificial nail should be taken off and once the nail is exposed to light and air the bacteria is killed. It does have to grow off, however. If you are getting a lot of these from where you are getting your nails done then you need to stop going there. It should not be happening on a regular basis.