I presume you have tried to unmount this datastore a number of times. I find that sometimes they will unmount from all but one host but then when I try to unmount a second time it then unmounts from the last host.
I vaguely remember having an issue recently where I could not unmount a datastore through the vSphere client but it worked from the command line with
esxcli storage filesystem unmount -l <Datastore-Label>