This is almost certainly Pseudoscleropodium purum - a naturalised moss. To confirm its identification you will need to look at the leaves with a magnifying glass or hand lense, at the apex, in this species the nerve (costa) extends beyond the tip, and then bends back on itself like a small hook. I think I can see this in your image - but you are best to check that yourself. If it is not this species, then its something in the Brachytheciaceae - for which - your are 'on your own'. You could also try getting an opinion from Dr Allan Fife (Landcare Research, [email protected]) who is our lead moss expert.