What causes nail greenies?

Greenies nails also known as chloronychia, are a nail infection caused by the common bacteria Pseudomonas aeruginosa. The shade of green varies from blue-green to dark green to bluish-grey. "Greenies" is typically limited to 1 or 2 nails and can affect fingernails or toes.
The nail is not painful; however, the skin surrounding the nail, including the cuticle, may be swollen, tender, or red.
The green discoloration on your nails is caused by the bacteria's waste. The stain will spread on top or under your nails also as the infection progresses.

Pseudomonas aeruginosa is exposed during "wet" activities, such as jacuzzis and contact lens solution.
Pseudomonas thrive in conditions that are warm, damp, and nearly with no oxygen.
Greenies can be avoided by washing hands and disinfecting manicure equipment.

Diagnosis

The nail is kept dry to prevent the spread of bacteria.
If the discoloration appears to be spreading, you should see a dermatologist. He can easily diagnose Green nails based on their clinical appearance. In case of any doubts, a nail sample can be taken for culture.

While the stain will most likely remain on your nail until it grows out, the green nail can be safely painted over.

Greenies are infected nails that should not be worked on.
To prevent the infection from spreading, the coating must be carefully and safely removed.
It must then be properly diagnosed by a medical professional, treated if necessary, and left until the infection has cleared.

chlorine bleach diluted 1/4 with water
Vinegar (acetic acid) has also been reported to be beneficial in this regard