I had this on the cockpit
window on our Athena. The black window sealant is eroding away, presumably due to sunlight on the edges and leaves a horrible mess when you wash the boat down. I gave it a really good scrub with a stiff bristle brush which erodes the sealant further leaving a U shaped groove between the window and grp and then used Black Marinflex (which is a 1 part polyurathane sealant/adhesive like some Sikaflexs'. After 3 years no more black streaks so far. The same problem is starting on the main saloon
windows, but less seriously as they are protected by the eyebrow. I plan to do those next year but will probably have to bodge up a shaped tube to get the goo in the right place.
Its a messy job, you can either mask up which is tricky, insert the goo, shape using a plastic tube and then remove the tape. Or, don't mask and wait until the mastic starts to harden and then use a paint
scraper/chisel to peel off the excess. Spraying with water
will accelerate the hardening