I'm facing a similar (if not the same?) issue. My tank is 2 months old and I test extremely often. I've been to all 3 reputable LFS in my city and have discussed this in length with my aquarium group.
The most common suggestion I receive is to not let nitrate sit at 0. I have dosed up to 1-2ppm only to see it at 0 the next day, but with the accuracy of my checker, it could be 2, just reading 0 within the margin of error. I dosed up to 10ppm nitrate today with potassium nitrate. Will check again tomorrow to see where that's at.
I have a decent-sized cuc now, and a sea hare. My old cuc didn't eat this type of algae, but they are fortunately not dying, either. I can see the next day where my urchin has been because he eats the purple off of my life rock.
I've researched this about as much as I feel I can. Following along because there doesn't seem to be an easy fix for these types of problems.
Here's my tank at the moment, it's usually blue but I wanted to see the algae situation (left side you can see some white rock that's been cleaned by the urchin).
Feeling pretty defeated myself. I research until I'm blue in the face. Testing multiple times daily at times to try and figure this out. I've not dosed anything beyond potassium nitrate to try and raise the nitrates a bit.
Here's my tank. I usually scrape the glass every other day but.... what's the point.