Greetings e2139,
As long as the System has not lost it's charge, the "green color" on a coil will not hurt it.
What you should have done on a yearly basis is have a HVAC Tech, "Clean & Tune" your system, which will involve pressure & leak checking, as well cleaning the Condenser & Evaporator coils, checking air flow etc.
And, as long as the Evaporator coil isn't leaking down in your Air Handler/Duct work via that rust, you should be good.
I will say, that after 20 years of usage,.... "If" You were concerned about Efficiency & Warranty, then it's time to upgrade to a new unit.