The only truly green energy is the energy you don't use. All electric is dirty to some degree. Hyrdo is cleaner than almost anything else, but not a clean as not using it at all. On top of that "clean" is not a very well defined metric. Carbon emissions are not the only thing to consider.
The exclusion of hydro from "green" is of tactical nature. It's mature technology and without spectacular learning curves.
I don't have the details, but when my company went to purchase RECs for 2021 we were told that hydro is no longer considered to be in the level of REC we were targeting (extra green something or other). So the cost for this year was much higher than 2020.
Dumb or not, hydroelectric has its own ecological problems and as the dams near the and of life, the costs associated with that will be most likely "externalized"
The exclusion of hydro from "green" is of tactical nature. It's mature technology and without spectacular learning curves.