They are a..organic biomass. As much as they take over organic life, even mechanical life, they also convert the environment. If you're curious for an example, search the mission "Cortana" from Halo 3, where you can see their conversation of the Covenant's former capital. You could also look up the Halo: CE Anniv terminals, which say that "single flood spore" thing. It's also debatable in whether Oryx would be able to take something like the Flood while a Gravemind is present. Without a gravemind, I'd agree he could, the flood becomes animals in that state. However, with a Gravemind, such a strong, collective conscious would be doubtful that it could be "persuaded" by oryx's method. It would also bring into question whether the environment flood could be taken- and in Halo at least, nothing with exception of Glassing or the activation of Halo has seemed to fully stop the flood at a particular place.