Cirrocumulus clouds are made almost entirely of ice crystals. According to the World Meteorological Organization’s International Cloud Atlas, strongly supercooled water droplets can occur in them, but those droplets are usually replaced quickly by ice crystals.
Cirrocumulus is a high cloud that looks like a thin, white patch or sheet made of very small grains or ripples with no shading. Because the cloud is so thin, a corona or iridescent colors can sometimes be seen around the sun or moon through it.
Source: World Meteorological Organization, International Cloud Atlas: Cirrocumulus, physical constitution
Image: Cirrocumulus clouds over Bergsfjorden, Senja, Norway, Ximonic (Simo Räsänen), CC BY-SA 3.0, via Wikimedia Commons.