While natural sunlight on it's own doesn't necessarily cause it, almost all captive systems have DOCs and other nutrients in above-natural levels that, with combined with enough light, can cause it.
That was my point, the bulb itself doesn't cause nuisance algae, phosphate/nutrients do. NSL is what corals in the wild work with of course, granted their depth in the ocean filters out certain spectrums, but that only occurs when you start to get deep where most of our corals don't grow. Organic's/nutrients of any measureable level in any system/with any bulb will promote nuisance algae.
And an Iwaski 6700K is equivalent to sunlight at noon, and of course it grows corals like no other bulb will.
The following tank is by far and away the best looking tank I have ever seen, and it incorporates NSL.
Google: Pieter van Suijlekom's reefkeeping