Equivalent of 'open mic', but for web camera

From a clarifying comment under the question:

What I am looking for is something which separates the 2 expressions: after all, 'open mic' refers to only the microphone.

While I like on the air as an umbrella term, I suggest the following more specific term for a webcam (or any camera):

Do not disturb

open mic, live video feed

What makes this distinction important is that it's possible you could decide you don't mind being seen in the background, but you don't actually want to have anybody hear you say anything. Without specifying both audio and video components, it might be unclear exactly what is involved.

The use of live suggests danger, or at least the need for increased awareness. An umbrella term also involving it would be live broadcast.

On air or On the air.
From Meriiam-Webster unabridged:
(1) : the medium of transmission of radio waves; also : radio, television
— often used in the phrase on the air
He went on the air with the first of a series of Saturday-night broadcasts — Atlantic

Though the technology has changed, the idiom has stuck. Much as in the way you might still say "we set sail" despite not being under wind power, or "crank the engine" despite there no longer being a hand-operated crank.

"Open mic" does not mean what you think it means: See https://en.wikipedia.org/wiki/Open_mic

"An open mic or open mike (derived from the expression "open microphone") is a live show at a coffeehouse, nightclub, comedy club, strip club, institution or pub at which audience members who are amateur or professional may perform, often for the first time, or promote an upcoming performance, are given the opportunity to perform onstage."

You need "On Air!"

For "Do not disturb!", I would have "Keep out of camera."