The authors of the paper uploaded 6 videos in the supporting info. It looks like mucus to me, but it is difficult to see. I cropped one video in which you can see strings of the "milk": https://i.imgur.com/9RvVSgz.mp4
Someone finds an organism and considers it to be "different enough" from all other organisms that have been described in the literature. This person will collect (or, in the past, sometimes illustrate) the organism and store it somewhere such as in a museum - this is the "holotype". The person will then write a paper with the description of the organism, compare it to some of the most similar known members, and make an argument for calling this holotype a member of a new species. If the species has particularly unique traits, or substantially different genetics, the author can argue for the description of a new genus - or even a higher rank.
But... The line is indeed extremely blurry. There is no universal agreement about where to draw the lines. The description of a new taxon is an argument, and experts disagree continuously. The tree is being continuously shuffled and it is not uncommon to see different publications using different scientific names for the same species.
They do, in many cases the choice to assign two populations to different species rather than one is not clearcut. All sorts of weird stuff can happen, like ring species around lone mountains :
In bacteria iirc these concepts still exist - mostly - though there is definitely a Ship of Theseus argument that could be made there.
For viruses though, woah! Literal chimeras exist with like the head from one source, the body from another, and perhaps the tail from still another. After all, why not? They are just functional nanoparticles, and on an evolutionary timescale of billions of years, pretty much anything that can be done will have been tried, multiple times.
These concepts are still practically useful though - well, as much as any concept is, given how the entire field of biology exists in a more or less fluid state. Literally the only law there is Virchow's stating that all cells come from previously-existing cells (by division), and that one even entirely on its face has to be false b/c what about "the first cell"?
Likewise the central dogma that information flows from DNA through RNA to protein has been proven false so many times it is ridiculous. Nowadays we know that it sometimes goes backwards, or skips steps, or both - e.g. proteins straight-up manufacture DNA from scratch (telomerases), or like proteins can make other proteins (not just prion conversion!).
Biology is... weird. It is definitely not just physics, or even chemistry, enacted on a larger scale - it's on a WHOLE. NOTHER. LEVEL!:-P
Herpetologist Carlos Jared of the Instituto Butantan in São Paulo and his colleagues have been studying these eccentric animals for years. In previous studies, the team noticed that ringed caecilian hatchlings, which live their first two months out of the egg in their mother’s care, spent much of their time around the end of her body near the shared opening of the reproductive, digestive and urinary systems — an anatomical part called the vent. The female would periodically expel a thick fluid from the vent, which the young would enthusiastically feed on.
Something I didn't quite understand after watching the video is in what way the "milk" is expelled or distributed, because maybe it's because of the camera's perspective but from what I saw in the video the babies stayed on top of the tail, not underneath, where I guess the vent is located. Could someone explain if I have misunderstood something?
In one of the videos this is clear. Some people might feel uncomfortable watching this video, so I will place a link to it under a spoiler tag just in case.
Amphibian and semi-aquatic aren't the same thing. Frogs are amphibians because they start as gilled tadpoles and turn into hoppy lungy frogs. Turtles are not amphibians because they hatch as a turtle, live as a turtle, and die as a turtle. It's turtles all the way down.