The Hidden are a parasite faction. They occupy foreign planets without their owners knowledge. The only way early in the game to know is that the planet shows less space for population than would expected for it's size.  They automatically obtain their hosts technology for colonizing planets, so they might migrate to other planets in the same solar system that other host factions do not have technology for. They travel as stowaways on foreign (both colony and military) ships to spread to other planets and host factions.

They cannot build on planets, but they do consume little resources from their host planet. Only experience players or highly economic specialized heroes can detect that resources are leaking.

The Hidden have multiple starting points in the galaxy. Usually attractive solar systems near the other faction starter positions or near the center hub where scouting ships are likely to pass.  They need a host faction to increase population, but they do research from the start even on starting planets and on foreign ships. Technology a host faction posesses is more easily learned and the effect is also spreads to other host factions. 

They breed slowly to avoid detection and can occupy no more than 20 % of a planet until they discover more advanced technology. At that point they may choose to be either be remain hidden to the host faction and disguise themselves as their own population, or completely reveal themselves which would either trigger local guerilla warfare on those planets, or a lead to a peaceful coexistence. They can be an advantage to their host factions by offering espionage services.

Only with advanced technology they can be detected. Other factions may decide to ally together to exterminate the Hidden van the galaxy. However it is also possible for a faction to ally with the Hidden to fight their enemies from the inside. The hidden also need advance technology to fight other factions, including other Hidden factions. This allows unique game play and strategies for this faction, and other factions which they encounter.

First, I like the concept a lot. Very creative. But I am a little concerned how fun this faction would be to play because they are so passive: they try to remain hidden from other factions, they travel by latching onto other faction's ships, they can't build on planets. So what will a player do on their turn, besides just hitting the "next turn" button.

I feel this concept could be interesting and implemented through a game event or a minor faction, rather than a major faction.