I am wondering if this is a special type of algae or if the water balance is being pushed over the edge for scaling because of the algae giving off CO2 in their normal respiration and causing the the scale to encase the algae, which I suspect is the scenerio. I can see this happening if the pH is already high and the calcium is high. The localized increase in carbon dioxide in the water could be converted into bicarbonate under the right conditions and cause a localized scale deposit around the CO2 'bubble' and create what seems to be a 'grain of sand'. I have been searching and searching and cannot find any references to fresh water calciferous algae although it is very common in seawater aquariums and coral reefs.