That's medium format film? I was confused by the picture and thought it was sheet film. In that case I'm back to my original advice: rinse it up in distilled water, finish with final rinse and see if the stain would wash away
Yes, ideally the tank should be level when processing. You may want to adjust water in the processor to make sure the drum is more or less level. It's less critical with film processing because you add a lot of chemicals, but when processing paper, the amounts are rather small so you may get bad coverage.