The Unstable Planet is a planet that has very active tectonic plates, which is one of many planet types and, one of 2 extreme planets currently not in Galactic Civilizations III and is one of two extreme planets that wasn't in Galactic Civilizations II.
How to Identify?
Unstable worlds can be identified by the unstable icon next to the planet and when hovering your mouse over the planet, it will indicate it's a unstable world. Unfortunately this planet type is currently not in the game, however, is expected to be added either at or shortly after release either by patch, DLC (Downloadable Content) or expansion.
Unstable worlds are very active planets with volatile tectonic plates that make it hard for any industrial race to build or erect permanent structures on the planet, of any sort. Unfortunately, for most races this makes it a planet that most industrial based life forms have little to no interest in as everything that they build is quickly destroyed by frequent earthquakes. Because of this it will be one of the extreme planets that require additional technologies or racial abilities to be able to survive in this environment.
Unfortunately no strategy exists at this point, however, one is to expect that like the other extreme worlds it would be best to rush to the technology to unlock it as quickly as possibly in galaxies crowded or with limited options for expansion.
Which races able to immediately colonize?
Currently there are no races that are able to colonize this planet type at the start of the game, however, it is possible that a race would be added in the future in a DLC (Downloadable Content) or expansions that this will become possible.