Star jelly is a semi-mysterious gelatinous substance allegedly deposited on the ground during meteor showers. The phenomenon of star jelly has been observed since at least 1641, and probably much earlier. In Welsh, star jelly is known as pwdre sêr ("rot of the stars").
A long 1979 article in the paranormal Fate magazine asserted this substance has an extraterrestrial origin, and constitutes "cellular organic matter" which exists as "prestellar molecular clouds" traveling through space. Some paranormal enthusiasts have drawn a connection between star jelly and the idea of atmospheric beasts, calling the jelly the remains of these animals.
Scientists are extremely skeptical, favoring a terrestrial origin for the substance. The scientific explanation for star jelly is that stargazers witness a meteor shower, then run in the direction where they think they fell, only to find a pre-existing slime on the ground, be it slime mold, nostoc, or lichen. Nostoc, in particular, a fresh-water cyanobacteria, has the potential to rapidly form colonies on open ground, appearing as a mysterious slime. Amusingly, nostoc is edible, being rich in protein and vitamin C, and is cultivated in China, Java, and Japan for human consumption. So "star jelly" may be edible.
In reality, meteors barely ever make it to the ground. Most burn up dozens of miles above the surface. Keep in mind that meteors are usually made of rock or even iron — if they had a jelly element, it would be incinerated by the outermost layers of the Earth's atmosphere. When it became obvious that "star jelly" could not be connected to meteor showers, paranormalists tried to connect it to molecular clouds, an even less likely source of the material.
Molecular clouds do indeed exist — they are observed by astronomers regularly. However, these molecular clouds are often very diffuse -- thousands or millions of times more diffuse than air, not to mention they are located many hundreds or thousands of light years away. Any molecular cloud of appreciable size would be observed by astronomers blocking starlight long before it reached the Earth. Even small molecular clouds would be observed by astronauts on the Space Shuttle or the International Space Station, but none have been sighted. Our solar system actually has a matter density greater than that of any prestellar molecular cloud, as our solar system is the outcome of a molecular cloud that has collapsed under its own gravity. Our solar system is scattered with dust, but none of it in jelly form.