For what it's worth. No ill effects from daily dosing. The blue colouring is ending up in the skimmer cup.
I was checking things this morning and for the second time I have seen this rather amazing thing. The sock that fits in the skimmer contained a fluoro fuschia residue which is a non-slimy colouring (not Cyanobacteria) that requires bleach to remove it from the sock. It appears to be a coralline algae growth event that causes widespread and rapid growth of coralline on rocks, and in this case, an amount is getting trapped in the sock. It appears overnight, along with a large burst of the purple algae. It seems the lanthanum is aiding the system to reach an optimum level for this growth. It's all very odd- but as I said, this is the second time it has happened. I was in partial denial...but the algae which I have struggled to grow, has exploded rapidly.