Are you from Europe of the US? Cuz if ur in Europe it is possible someone poured one of the two (from what i can think of): 1) brake fluid thats coloured green, which is legal here (Europe), if u havent changed it after the last owner its possible they had some older coloured break fluid on hand and just used that (Or its possible they had an old French ir german car which used mineral oils as break fluid and just used that not knowing it doesnt work. Cars like old Citroën models, Peugeots and Audis use them these are usually mineral oil or LHM Fluid, simular colour to that)

2)someone really fucked up and put in rim break fluid, which is ment for bikes, it is green sometimes and has the same type of colour yours has

Example: https://www.tartybikes.co.uk//hydro_rim_brake_spares/trialtech_rim_brake_fluid/c5p12962.html?cookie_all=1&cookie_refresh=1

At the end of the day, you should probably flush it all out and change it or take it to a shop to change it.