I think the general consensus is that "moonlights" have no ill effects on either fish or algae.
Having said that, I've read a number of studies which suggest that exposure to 460nm light (I.E. the blue lights many people use as moonlights) directly effects circadian rhythms in both humans and fish by interrupting the uptake of melatonin. Such studies (the ones I've read) further suggest that cognitive reactions improve after such exposure. The inference therein being that sleep is disrupted and that long-term exposure or exposure at the onset of the circadian cycle can lead to sleep disorders.
True? I dunno; that's not my field.
In any event, I choose to use moonlights for a few hours at night and in the morning, but make sure the tank goes dark for at least 7-8 hours every night.