Healers have to heal the mushrooms to full to activate them, then keep healing them to keep them alive. They need to not do it too early, like that shaman tard we pugged on Saturday kept doing, because the shrooms get automatically debuffed by "Withering" and "Withering Rapidly". They can be kept alive during Withering, but once it switches to 'rapidly' it's over.
Green one heals the raid through the Infesting Spores AE, blue one is haste and mana regen. Tanks' job is to be aware of the mushroom spawns and drag Bracken over to them, while also making sure that he doesn't breathe on them.
One thing not immediately obvious is that the mushroom buffs are actually a 20 yd radius, which is much larger than the AE graphic they place on the ground.