Hi Russell ,
Yes, in the P.linkeii tank the white structure is near the nest, I also thought there could be a connection.
But in my P.phoenicurus tank there is it also, specially very heavy under the foam material on the tube, where the water goes up into the rucksack filter.
Perhaps there lives micro food for the larves? And it’s special in sour water/black water?
The (I know now it’s the bigger one) male has chosen the place for his “camping” perhaps because there in the corner comes most of the food with the slow water stream, an beside is a jungle of Java moss where the little fish can hide.