Can you get some lab grade isopropyl alcohol?
Sometimes the best thing to do is not to touch it at all, but just get a few drops of alcohol to flow over the surface.
I use a syringe with a needle to squirt it at the optic.
If the alcohol is clean and pure, it will evaporate with no residue.
Hopefully, whatever the mark is on your dichro will disolve.
James.