I'm sure you have enough trace PO4 in the system to grow algae on the screen. Mine has measured zero, using salifert, for nearly the entire time I have used a scrubber
and I've never had trouble growing algae, cleaning two screens weekly. Even started up a couple of new screens with zero PO4.
In regards to the blue lights you're using, they won't be helping at all.
People who use blue or violet use it in a ratio of 4 red to one blue/violet, & they run the blue/violet at half power.
Its known by these people that a single blue or violet, at equal power of the reds, prevents algae growing on the screen directly opposite the blue/violet.
Get more reds
and I've never had trouble growing algae, cleaning two screens weekly. Even started up a couple of new screens with zero PO4.
In regards to the blue lights you're using, they won't be helping at all.
People who use blue or violet use it in a ratio of 4 red to one blue/violet, & they run the blue/violet at half power.
Its known by these people that a single blue or violet, at equal power of the reds, prevents algae growing on the screen directly opposite the blue/violet.
Get more reds