The only way to do it correctly, in my opinion, is to scrape it all off, clean with thinner, and start over with Dicor. That looks like silicone, and nothing sticks to it well.
If the antenna is leaking, I would remove the antenna and its gasket, clean and inspect it all, and then seal the gasket to the roof and the antenna to the gasket with a layer of butyl tape, tighten it down, and then top with Dicor,
2014 Sunstar 31KE
1988 Suncruiser 31 RQ
1968 Travco 210