Not a disease. That's a mould which forms on dead and dying tissue. Those are phormium leaves which look to me as though they've succumbed to the year's hard conditions. A cold winter which rotted plants, and then drought which finished them off. Phormiums are New Zealanders which won't tolerate cold and wet. Clean away the dead leaves, and keep the plant fed and watered in a well ventilated position (west facing) and give it time.
Phil J

Here I was thinking it was mealybugs! I would have to get a closer look to be sure though.