Plan A: Leave it soaking in isopropanol (SU-8 is insoluble in it) and it might swell the PDMS, so it'd be easier to remove. Unfortunately, I wouldn't be too optimistic about saving the silicon master.

You should first try to pour again PDMS on it and peel it off. There is a chance to capture the residues on the newly cured PDMS.

If it doesn't work, you should check this paper:

Conference Paper Removing Cured PDMS from Micro/Nano-structures

I never tried the proposed method but according to other RG members, it works.

(https://www.researchgate.net/post/Does_PDMS_have_any_solvents

Other ressources: https://www.researchgate.net/post/Which_solvent_dissolves_PDMS_completely

https://www.researchgate.net/post/What_is_a_good_way_to_remove_cured_PDMS)

Good luck!

Of course IPA or ethanol should be the last washing step, but maybe you should try a preliminary detergent step (using SDS 1% in milliQ water). I would also do what Theo Aspert mentioned first with tape if there are some solid pieces remaining