Soluble salts can build up to damaging levels in float trays. Although these salts could be naturally occurring in the float water, it is more commonly due to salts in the fertilizer. As water is pulled to the surface of the float trays, fertilizer salts come along for the ride. As the water forms a film over the dividers between cells, it is susceptible to evaporation and as it evaporates, the salts are left behind. By running a finger over the divider one can feel the salt crystals. The color can vary some but is usually a brown or brownish yellow deposit. Damage can occur at the stage seen here or later as leaves grow bigger and touch the salt deposits a salt burn can occur at the point the leaves touch the salts. While soluble salts injury can be mistaken for heat related stress, note that the algae appears fine and should show heat damage also if that was the main culprit.