Why is my parakeet's beak green and looks like mossy growth?

Without seeing a photo it is hard to tell what you are describing. Normal aging can cause the beak to change color as a bird matures. Injuries to the beak may cause bruising or discoloration, malnutrition or other illnesses may also cause beak color changes. If you're seeing any other signs of illness, it is always best to seek veterinary care to have the beak looked at and determine if it is an issue.

Beak color changes can sometimes happen in parakeets when they are ready to mate, or are near a female that is currently ready to mate. If he is housed with any other birds this could be the case. Some illnesses and infections may also cause beak color changes, but you may see other signs in addition to this such as changes in appetite, stool, energy, etc. I would keep an eye on the beak if he is still acting normal, and if you see any signs of illness, or the beak becomes bothersome/prevents him from eating and drinking normally then scheduling a vet visit is best.

If you're seeing white lesions in his mouth or on the inside of his beak it could indicate an issue such as a bacterial or yeast infection that has taken hold. If you're seeing streaking on the outside of the beak, it could indicate a cracked beak from an injury or even poor nutrition/underlying illnesses. If it's impeding his ability to eat or the lesions are on the inside of his beak/mouth it is best to seek veterinary care to try and find the underlying cause and get treatment. Making sure Kiko is on a diet that is formulated for parrots can also ensure he's getting the right nutrients. You can offer fruits and vegetables as extras, but a balanced diet is needed for optimal health.

Beak overgrowth can be caused by a number of things, and if the beak is continuing to have issues or strange coloration with it, it would be best to have it checked out by an avian vet in your area. Giving your bird access to cuttlebone or other items to wear down the beak with can also help prevent overgrowth, but ruling out any health problems first is best.

Some crusting can happen when a bird is ready to mate, but that looks a bit more than that -- It is possible your bird has a respiratory issue going on, a fungal infection taking hold in the nostrils, or another type of infection/illness. It would be best to bring him to a vet to have the area flushed out and treatment started for any underlying infection. Separating him to a hospital cage that is kept warm and with easy access to food and water may also help to prevent the spread of any illness and to keep Ocean comfortable if your other parakeet is picking on him until you can be seen by your vet. You can also use a warm washcloth to try and remove any debris to make him more comfortable.

Some infections such as fungal infections can cause flaky growths around the beak or nose. Growths on the skin from infection, tumors, parasites, etc may also cause a dirty growth appearance anywhere on the body. Regurgitation of food may also cause stuck debris, as well as problems with the crop causing food or bacterial to come back up into the mouth. If it's impeding Chico, can't be cleaned away (or returns even with cleaning) or you are seeing other signs of illness such as changes in energy levels, behavioral habits, eating and drinking habits, etc, then having a vet look at it in person is best. They may need to flush the area and clean it and start Chico on a round of medication to treat as needed.

Your parakeet could be suffering from a mite infection caused by the bird mite cnemidocoptes, The infection would show up the beak as well as the legs. You should have your bird seen by a veterinarian who can easily diagnose a mite infection and then treat it with ivermectin, You should bring both birds, as both need to be treated.