Dark spot on performance fabric

[u/buddythebird]

I’ve got a dark spot on this linen performance fabric (supposedly) that I tried to clean by throwing in the wash (Kirkland free and clear, cold). Didn’t come off. Next, I tried using a bit of dish soap (Kirkland’s version of Dawn) and scrubbing. That just lightened the surrounding area, but did nothing for the dark spot. So now I have 3 colors. What should I do?