Sounds contaminated to me, brushes that aren't used very often lead to ageing liquid in the hair (usually this is yellow).
Putting a lid on a brush (without having a decent airing/evaporating hole in the lid) can result in the liquid going off in the hair.
Brushes need to be well drained (as near to dry as possible) before putting a lid on them.
The colour "green" would be some kind of metal issue/oxidisation, rather than old product, ....... to me, it would mean that the ferrule of the brush is made from copper, because old or affected copper turns green.
The more you use your brush, the longer it will last (as it is being constantly renewed with fresh liquid).
I shampoo and condition my brush every 2 weeks and allow it to dry over the weekend before use.
Just to add, I have NEVER stored a brush hair down in 29 years, maybe longer, in those years I have only had 2 contaminated brushes, and that was because of cheap crap glue used to glue the hair in. One of them, the supplier disputed my comments after 4 weeks of use, yet within 6 months they changed their whole brush range and supplier .... that particular brush had a look that was synonymous with the brand, I now use that "obvious looking brush" to demonstrate brush contamination , in that short period of use they should have refunded (no questions asked)