I would try using a single-edge razor blade to scrape it out.
Silicone is used to make all sorts of bakeware, so I don't think it poses a health risk. Try Googling and see if you can find anything about it. Certainly, if you can scrape it out, then it should be safe.
If you don't have to save the seasoning on the cast iron, then remove everything using electrolysis. That might take the silicone out without scraping, but it will also remove the seasoning down to bare metal, and then you'd have to re-season it immediately or it will rust.
Electrolysis - Rust Removal