Mealy bug maybe?? but, hopefully not!
Provado bug spray maybe useful if it is that ! Is there a sticky substance at the bottom of the leaves?? Might be worth checking?

Phormium mealybug is really about the only thing they suffer from. Malathion used to deal with it, although it needed treating every year, but that's been withdrawn. However, the brown marks on the foliage are not uncommon in spring, its usually the result of winter, so I'd trim off all unsightly leaves at the base and the new growth should come through just fine. Mealybug infestation tends to reduce the vigour of the plant rather than producing the brown marks. Better not fed if in the ground, this is one plant that does better without feeding, although a top dressing of Growmore raked into the soil roundabout wouldn't do any harm at this time of year.