It's not 'frazil ice'

Technically, frazil ice is a slurry of ice particles that forms at the supercooled surface of turbulent liquid water.  Frazil forms in rivers and in wind-blown lakes/reservoirs in cold places.  What we have in Yosemite is a slurry of ice particles that forms as liquid water is aerosolized in a waterfall descending through sub-freezing air.  The end product of each process looks similar, but the way the very different talus and till can look similar or the unrelated heron and a crane look similar, to be proper, frazil and the distinct Yosemite waterfall feature should be distinguished with a different label. 
We might shorthand this frozen waterfall mist as 'wist ice' or a 'wist slurry.'  Because its origin is distinct from true frazil ice, our phenomenon deserves it's own name, though I wouldn't really expect our established common usage of 'frazil' to change.
One line of evidence that tells us that this isn't frazil ice is that when the creek below the waterfall is filling with the slushy mass, there is no ice above the falls.  This photo shows Yosemite Creek just above Upper Yosemite Falls at 7:30a.m. on 16 April, when it was 24 degrees F there.

There are no ice particles above the waterfall, but a dense slurry flow below.  No supercooled liquid is involved.
We've had wist slurries flow in Yosemite Creek and Ribbon Creek the past couple of mornings when temperatures dropped into the 20's.  Near-term forecasts don't look cold enough to produce much wist for the next few days.
Yosemite Conservancy's Monday and Thursday 8 a.m. birdwalks in the Valley have resumed for the next two months.  Orioles and grosbeaks have arrived in Yosemite Valley; we eagerly await our first tanagers.  The first warbling vireo of the year was heard yesterday.
The Hetch Hetchy Road has re-opened after repairs.  Merced River Plan public comments are due April 30.  Tioga Road is melting out faster than usual.  River and stream run-off has been higher than usual for the past two weeks, but it's expected to taper off early.  Seasonal waterfalls should be quite small by mid-summer.
There's still space in Dave Wyman's 'Spring Light Photography' course in mid-May.  Dave is a charismatic teacher with years of experience in Yosemite, and Yosemite always has something to teach us.