Here in PA, and I'm speaking as a criminal defense lawyer, this could
be a simple assault (and possibly an aggravated assault, depending on
the circumstances), a crime called "recklessly endangering another
person" and possibly several other charges as well.
The claim that the thrower did not see the cyclist could be a defense
to the charges, but it does not mean that the alleged act does not fit
the definitions of those crimes. That likelihood could, however, be a
factor in the official decision to bring charges or not, although at
that early stage the authorities wouldn't know that.
To the OP, you might want to look into whether California has a
"private" charging mechanism, which in PA, is called a "private
criminal complaint." Regards, Roy Zipris