First start with a new toothbrush and scrub with alcohol, that removes most of the gunge. They normally are nickel plated brass, so generally can be cleaned up reasonably. If really green and the alcohol does not remove it then you can use Brasso ( brassware cleaner) with the toothbrush, rinsing it off by scrubbing with a new brush again with the IPA to get the stuff off of all the crevices and the inside. When clean coat with light lubricating oil (3 in 1 works well, I use a synthetic that I use otherwise at work) to protect it. Best is to place a terminator always on it, or a plastic dust cover.