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 are usually a brown or brownish yellow deposit.  
Damage can occur at the stage seen below or later as leaves grow bigger an touch the salt deposits a slat 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.

(Pictures  courtesy of Paul Bachi)