[gorillarick]

I know nothing about Nova.

Black ring sounds like a carbon (charcoal) filter collapsed/leaked into the water.
Should look like very fine black particles.

Definitely won't kill you, but I wouldn't make a habit of it.
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If you suspect mold/bacteria: Add a cup of bleach while still in there. If bacteria, it will seem to disappear.
(oh, and don't forget to flush your toilet before leaving for any length of time)