I wouldn't want that in my brakes. The marks from the pliers would allow fluid to seep past and probably rip the seals to shreds. Result - no brakes.
The only consolation might be that it looks like there was pitting and corrosion on the surface - which would have meant replacing it anyway. It's only 30 CAD (ish).
For future reference - there are tools that grip on the inside of the piston to remove them - not that I have ever used one, so I can't advise. Compressed air is excellent - but needs the fluid to be removed first and a bag in which to catch the flying projectile.
If you don't have access, then the best bet is to use the power of the brake system itself and slowly pump the pistons out. It's possible that one will come out faster than the other (in twin piston units, or the outer psitons in a 3 piston system) - if so, engineer it so that the reluctant one gets pushed out, preventing the first one from moving until they are both moving out together. One will pop out first, leaving the other to be removed by hand, but this way you are not left trying to get the more difficult one out by hand, and it will be close to dropping out anyway.