jebussaves88 said:
Spores is the actual term I believe. I tend to abbreviate to "little fuckers".
The official term for them, used in all Halo media (novels, graphic novels, games, Bungie website, etc.) is Infection forms, or Pure Flood as this is their true form. The larger ones with weapons are called Combat forms, and are the effect of an Infection form taking over a host's body. The fat explosive ones are Carrier forms which are created when you leave a Combat form for too long and it evolves to become an incubator, containing more Infection Forms. Simple really.
EDIT: When fighting them I usually call them little bastards, just as I call the flying Drones of the Covenant 'buggers'.