PSA: Green staining is NOT fungus
I wanted to make a post to clarify a few things regarding greenies, because I see a lot of misinformation repeated all over this sub (sometimes even by other nail techs/professionals) on a daily basis, and it can really scare people to think they have a fungal infection.
Greenies (chloronychia), aka the green staining you can find on your nails if you have unaddressed lifting for a long enough period of time, are not fungal infections.
You aren't going to get a fungal infection solely from moisture getting trapped under a lifted nail enhancements. Fungal infections are spread via fungal spores, most often shed by an infected person. You have to be exposed to the spores to develop nail fungus.
Fungal infections are very often spread from contaminated nail tools or workspaces. Salons that don't properly disinfect tools, lamps, footbaths or desks between clients, or salons that reuse towels, nail files and buffers instead of using a fresh one for every client are one of the biggest risk factors in getting a fungal infection. You can also pick it up from places like gyms or public pools, showers/locker rooms, sharing towels with someone, etc.
Trapped moisture under a lifted enhancement can cause greenies, which are a bacterial infection that leads to unsightly green or yellowish staining of the nails. It's caused by pseudomonas aeruginosa bacteria reproducing under a lifted enhancement, where the environment is warm and moist and perfect for bacteria to proliferate. This bacteria lives all over the place, it's very commonly found in water or soil, and all it takes is a little bit of lifting for it to get in. The waste that this bacteria produces is what causes the green or yellow staining.
It's a good idea to see a doctor if you're concerned you have a fungal infection or chloronychia, as nail techs are not allowed to formally diagnose these conditions. In most cases, greenies look like normal nails with staining ranging from very dark green in severe cases, to mild yellowish staining that's only on the very surface in mild cases. Fungal infections tend to look more whitish or pale yellow, and the nails tend to be thickened, ragged, brittle and crumbly, and it often causes the layers of the nail to separate from each other, or the nails can even detach from the nail bed underneath.
Greenies generally do not require medical intervention except in very severe circumstances. It used to be believed that it was safe to cover them with new enhancements as the bacteria dies upon exposure to oxygen, but those teachings are now considered outdated. The most recent up-to-date recommendation is to remove enhancements and keep the nail bare until it grows out, after which it's safe to apply new enhancements. Greenies can happen even to the most experienced, professional nail techs, and this is one reason why it's important to keep up with regular fills/maintenance and not push it too long between appointments.
Nails with fungal infections do require medical treatment, fungal infections don't go away on their own, and they should not be worked on in any capacity by a nail technician, they should be treated by a medical doctor or specialist. Fungal infections are very hard to treat, they're very contagious and it's a more serious condition.
Greenies are a lot less scary than they look, and in the vast majority of cases, they're harmless, they grow out with the nail and they don't cause any lasting problems. The best way to prevent greenies is with proper nail prep, regular maintenance/fills, following aftercare instructions (don't use nails as tools, wear gloves when washing dishes, etc.) and removing or fixing any lifted product as soon as you notice it.