iPhone 13 camera showing a green dot.

That is definitely lens flare and SravanKrA provided an excellent explanation of what lens flare is. Before you took the photo, the flare was plainly obvious on your iPhone screen. Reangling the camera relative to the bright light you took a photo of can minimize or completely eliminate it. There is nothing wrong with your iPhone or camera in your iPhone.

Does this green dot appear on every photo on the same spot and same size? If no read the Wiki article below. If yes then clean your camera surface and if that does not resolve the issue then Contact Apple for support and service - Apple Support 

Lens flare refers to a phenomenon wherein light is scattered or flared in a lens system, often in response to a bright light, producing a sometimes undesirable artifact within the image. This happens through light scattered by the imaging mechanism itself, for example through internal reflection and scattering from material imperfections in the lens. Lenses with large numbers of elements such as zooms tend to exhibit greater lens flare, as they contain a relatively large number of interfaces at which internal scattering may occur. These mechanisms differ from the focused image generation mechanism, which depends on rays from the refraction of light from the subject itself.