It is called a scintillating scotoma or ocular migraine. Scintillating is accurate; this clear crescent shows up in your vision and It is rippling with rainbow fringes. It starts off as a point somewhere in your vision, slowly expanding into a visible crescent that continues to expand. After about half an hour the crescent expands beyond your field of vision (FOV). For me it is paired with a loopy slightly lightheaded feeling that goes away with a whoosh once it leaves my FOV.

They are often triggered by bright light and may be correlated with migraines. I’ve had three of these happen but never had a migraine headache afaik.

Edit: clear might be the wrong word? My mind blends it in other than the rainbow fringes, but I think I can’t actually see what is behind the crescent.