9 hours is wild, but I read somewhere that the oils in coffee start going rancid pretty fast once it hits air, like within a few hours. That crust is basically the grounds drying out and the fats turning, which is why old coffee tastes kind of flat and papery. In my experience a french press is worse than a drip machine for this since all the grounds sit right in the water the whole time. Take it with a grain of salt since I'm no chemist, but yeah, that layer is a decent sign to dump it and start fresh.

Rags under every sink trap before I touch anything.

A filler won't do much for green hair tbh, that's more of a color correction job. Green usually means the lightener grabbed too much and the cuticle is shot, so you need to pre-pigment with red or copper first, THEN tone. Comping her service was the right call though, and honestly refusing Saturday walk in color sounds like the only way to survive a busy book.