Well Sudesh,
If you have CO2 with Siphon then with using different nozzles, you can
try out to clean the mask from one way to other on mask as tank
pressure, it does clean nicely.

Second one if you have photo resist stick to mask then you can take
clothe clean room wipes and wet it with acetone and then wet the mask in
DIwater and then clean it from top to bottom.

After these cleaning you can use nano strip clean for 10 minutes.
Piranha etches very slowly the Cr, so better be carefull doing so.

Parshant