Yes, Crystal Plex is still effective in killing algae at the same application rate after it has been frozen.  Freezing affects the residual control for Crystal Plex. Crystal Plex normally has a residual control of over a month.  Frozen Crystal Plex will only have a residual effect in controlling algae for 3-5 days.  When using frozen Crystal Plex be sure to shake well before diluting and spraying algae for control. I recommend using warm water to dilute the frozen Crystal Plex before spraying.