I have an old hn white trombone that's about 95 years old. I got it back in 2019 and had it fully cleaned and serviced then. I rarely play it, but when I do I notice when I empty out the spit it's green. Is it from corrosion or does it just need a good cleaning? And if it is corrosion, what can I do to stop it?

Copper corrosion is green, so it seems very likely that's it. The only way to "stop" it is to keep it dry.

I would suggest having it chem-cleaned by a good repair tech - that will remove the current corrosion - and then try to keep it lubed and dry when you're not using it.

Sounds like a cool horn.