No, I have never noticed this because I don't touch it with my bare hands. Will be interesting to see what others say.

Hi Nan, I haven't had hair algae in any of my ponds for at least 25 years due to the filter I built but, when I would reach into the water to remove this algae, I don't remember it being cool. Some explantions that could be is that of course, the water is much cooler than your body temperature, obviously but it could also mean that underneath the hair algae the water would be cooler due to the amount of algae there is and the water underneath would be shaded from the sun.

Your's Koily, Lorraine

Thanks, I'm not worried about it, just thought it was kind of strange. It only appears to be the older clumps in the stream and the bees seem to like to gather around it. Maybe they can feel the cooling benefits of it.
Could it be I have special string algea !!
I wish I didn't have any but it isn't much of a problem. All part of nature I guess.

If I had to guess I would that the string algae is above the water level is probably evaporating water quickly thus cooling the algae off. If the air was moving right through the algae it could cool the algae down to the dew point.
Mike

I took some out of 72F water and wrapped it around a thermometer and it went to 68F in just a few seconds. Evaporation. This is how evaporate coolers work.