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.

No I've seen what you describe. What I see when I'm having one is most often a shimmering electric zigzag. Sometimes it take different shapes and the colors vary. Its always at the edge of my vision and moves when I move my eye. Its better to close your eyes when they are happening. At least for me. I have had them in both eyes at once and its really freaky since normally I only get them in one eye at a time. They don't merge well and the combined blind spots have rendered me almost completely unable to navigate. I've pulled over while driving more than once and waited it out.