Here is my $0.02 worth.
At first glance your Alk level is too low. Your calcium level shouldn't be a contributing factor.
What do you have for clean-up crew? I suspect that a couple more snails, some hermits and possibly a tiger-tail cuke would make short work of that.
Also, try some phosphate sponge. Even though your phosphate level reads zero, it could be that the algae is consuming it all so it shows as '0'. By using a phosphate sponge you will help to starve the algae of its main food source (light being the other).
Same attack method as hair algae. Reduce phosphates, high alkalinity, reduced photoperiod, healthy sized clean-up crew, regular (but not excessive) water changes.
It'll be a couple week eyesore, but should eventually run it's course with the above help.
