Yes, but with less light the hornwort can grow long and stringy, but that is OK, keep it alive and it will fill out nicely once the hot sun shines on the pond. You should also take a piece and get it growing in a glass jar of water by a cool window, or another fish tank, and then you will have more come spring.
Never give all of the hornwort at once to the fish as they may "pig out" on it come spring, If they eat lots, no matter how shabby and chewed the pieces may be, the piece's that are left will regrow like crazy once the sun is very hot. In the early spring when the water is still cool, the string algae may try to choke it out, cause the string algae can grow no matter how cold. I just hand weed the little bit of string algae out and baby the strands for the cool spring weather. But Come the hot summer sun, the hornwort will grow so fast it can choke out the algae very quick!
I have a very large supply now in my ponds, and it still looks like green, although in the ice cold water there is very little new growth right now. It winter's every year in all three ponds below the ice.
