I had the same problem with my Neptune washer until I ran a cycle with straight vinegar and water.  This helped get rid of the mold.  I have to run a "vinegar" cycle about every 10 cycles.  I would try cleaning the seal with vinegar first and let it air dry.  Neptune washers need to air dry after every cycle.

How old is the washer? Did you try straight bleach on a towel and placed over the "mold" spot for about 5 minutes, be sure to wear gloves. Then scrub with a small brush being careful not to splatter bleach on yourself and rinse well. May need to replace the seal. I always leave the door cracked open after every use to "dry" out the inside and the boot. I also leave the drawer pulled out when not in use for that area to dry out as well. I have never had mold on the door seal of my Neptune washers, only the water holes above the dispenser drawer.