Termite infestation
Termites are one of the few organic ship-sized species that inhabit this universe. For anyone who builds bases, these are a pest.
Infestation starts innocently enough with small, weak green termites. They wander alone and erratically through a patch of the universe until a galaxy suits their fancy. Their selection process is unknown, but they will only spawn a termite mound within a galaxy that has player-built bases. Once the mound is complete, the little wandering termite continues on its way.
The mound has four tiny arms that will grow as it gets stronger. It will generate termites of increasingly higher level that orbit around their birthplace. It is unknown what sets these guys off but after a number are ready, they assault as a group any player structures in the same galaxy. Only after some hours have passed or their complete destruction will they desist, until the next group.
The mound itself protects itself with base equipment of increasing strength as it matures, becoming as difficult to destroy as most player bases themselves. It can be done, but since the termite mound only has a lifespan of one or two days players generally leave big mounds alone, trusting that their defences will be more than enough to defeat any termite mob.
Eventually, termites start having resistances and weaknesses to differing weapon types, culminating in the highest levels being vulnerable to only one weapon type. To find out what weapon works best, observant players should note the colour of the termite is close to the same colour as the general weapon type colour (light blue for surgical, for example). For a general infestation it is a good idea to fill base weapon slots with mid-range weapons of types other than laser, or drops drones to fill up those weapon type gaps.