Very near the most massive body -- other than the star itself -- in a remote solar system, a tiny object appeared. Gravitational waves had presaged its arrival, so the sudden change in mass distribution had no affect on the trajectories of other bodies, not that a mass that small would have affected them much anyway. After a few minutes of thrust to stabilize its orbit, the tiny being became still, but turned inward and started to change.
Originally it resembled a minute grain, perhaps a thin piece of rice. After only a few hours the linear form had swelled to many times its size, and had extruded multiple varied appendages. Most of the new bulk was empty space, save for a pressurized gas which seemed to nurture the even tinier parasites that dwelt within.
As soon as it had appeared it had begun to call out. And heard a reply. Another being, although one without parasites, echoed the call and the two exchanged considerable gossip. The other being had been long in the system waiting, and directed the newcomer to the closest patch of fertile ground. A small asteroid orbited low inside the orbits of the large body's natural moons. It was not a stable orbit, but it had been maintained there by the other being, employing a range of devices to convert the rock to more usable resources.
The newcomer attached itself to the bit of rock and immediately began to grow. It extended a long tail in one direction and a spine in the other. The being along with the rock began to spin end over end, and the parasites reveled in the slight change in forces. Shortly thereafter it started to pulse with energy, and a stream of particles pushed it out of orbit. The newcomer, coaxed by the other being, had merged itself with the rock and was slowly falling into the inner system.
It would decelerate for many months, slowly transforming itself to please its parasitic masters, until it finally eased itself into orbit around the fifth planet of the system. By that time it had fully rooted into its bit of soil and had sprouted leaves and flowers of every conceivable type. It was also ready to bloom, and to send new seeds spraying in every direction. But it was not yet time to spawn.
While ready to unleash its fecundity our starship has restrained itself, or been restrained by its lack of resources, generating only the minimal required services and facilities to support the human mission here. Given the green light we'd spread unchecked, our technology acting like the ultimate invasive species in a virgin environment. The limitations of star travel made us build ships like seeds, but we don't use them that way. It's far more important to explore than to terraform.