Originally Hypnum uncinatum (Hedwig) which looks a pretty decent shout, then Drepanocladus uncinatus even until Watson (1985?), but now the less-easy-to-remember Sanionia uncinatus. Maybe once you know why it's easier to remember, but at least it's distinctive, even if the BBS field guide key is wrong - describes the leaf as circular with a 3mm diameter, when it must mean circumference since the species page says leaf length is 3mm (1 x pi). This matches what I have, so I'm assuming the species page is the correct reading.
Sanionia uncinata, Sickle-leaved Hook-moss