"Words change meaning according to the context"
In your context, favourites/Bookmarks are not the right choices for sure.
There are some really bright ideas right here in the feed "track" & "watchlist" being my favourites (ah ha)
Sometimes, it can be done using an action as well.
"Pin to watchlist" can be a button which allows you to add a video feed to a separate section (watchlist). Simply unpin to remove it from there.
Icons with a toggle state are another good way to achieve a somewhat similar effect
an eye icon with states enabled and disabled so users can click and toggle the state - which is basically adding or removing item from watchlist
If the number of all the available stream is very large then track or save would be a viable option. "Save to watchlist"
The flag doesn't sound good purely because of its history with "report as spam" context. Ultimately, I'd suggest going with what goes best with all the other aspects & features of your app. If you have the concept of channels, lists, playlists or anything special go with something that fits the bill.
This was more like an aggregation of the ideas mentioned above rather than a new idea itself. But it should do.