This is definitely mold/mildew as I have it on my trailer (Scamp 13) in same spot. At somebody's suggestion here last year, I bought a jar of Concrobium power from Home Depot. Mixed as directed, minimizing foam, and then poured as directed into pump sprayer. Let sit on the belly band silicone for whatever it directed (2-3 hours maybe, I don't remember). Then rinsed it off.
I have done this same treatment twice now as my mold was quite bad. After the second treatment the white silicone over the belly band has been dramatically improved! Drove me nuts!
Just be sure you get the power that you mix in a bucket, not the spray as it doesn't work. The one I got was called "Concrobium Mold Stain Eraser."
It did not harm my trailer's FG/gelcoat either.