When a greenish-black discoloration appears on the nail plate, it is most likely a ‘pseudomonas’, a type of bacterial infection, aka, "The Greenies". The greenies are caused from moisture being trapped between the artificial nail and the natural nail.
"THE GREENIES" are directly related to the artificial nail not adhering properly. If there is anyway moisture can seep between the artificial nail and natural nail, it can be the perfect breeding ground for the ‘greenies’. The breeding ground needs warmth and moisture (basic food for pseudomonas). So, the greenies are found on the nail plate and not under the free-edge.
"THE GREENIES", aka, a pseudomonas bacterial infection is not a mold. The term ‘mold’ has been used for over 20 years in the nail industry to describe, what has most likely been a pseudomonas bacteria. But ‘mold’ is not the correct scientific term. Whereas mold is a fungus and ‘the greenies’ aka, pseudomonas bacteria, is not a fungus, but bacteria.
"The greenies" can be easily spread, usually with un-sterilized files, nail dust from filing, cosmetic dusting brushes, and anything not cleaned before next client, but the greenies are easily killed.
When a nail has the greenies it is recommended to remove any artificial covering and expose the greenies to the air. Then apply T.T. ‘Fresh-Nail’ to the affected area for about 1 minute, either with a T.T. Towelette or full submersion of the affected nail.
The stain is permanent and will have to grow out. You should be able to apply a new artificial nail immediately, unless the natural nail is very soft, then you should leave artificial nail off for 2 weeks and check at the next appointment.
Using the T.T. Sanitize often during the day will help rid nails & hands of bacteria.