A visible proto-language is an ad hoc technical term used to refer to a full conlang that serves as the proto-language for another full conlang. This contrasts with an invisible proto-language, which is usually not fully fleshed out, and is not intended for public consumption. For example, the proto-language of Astapori Valyrian is High Valyrian. High Valyrian is a visible proto-language, in that it is also presented as a full conlang. The proto-language of High Valyrian, on the other hand, is Old Valyrian, which is an invisible proto-language. It's not fleshed out to the same extent, and is not intended to serve as a fully functioning conlang.
As a note, natural languages that serve as a proto-language for a conlang will not be listed here. For example, modern English is the proto-language of Trigedasleng, but neither it nor Trigedasleng will be listed on this page.