The size of the moss would be interesting. I assume it's tiny and water-repellent, isn't it?
It might be a moss that is often found on wet ground and even floating on water in greenhouses of botanical gardens, water plant nurseries etc. I also have found it on rockwool in pots with emersed aquarium plants.
I believe this frequent greenhouse moss is the pantropical moss Philonotis hastata
that is possibly synonymous with Philonotis evaninervis
. Has to be proven by moss experts.
Mentions in botanical literature: http://www.buxbaumiella.nl/pdf/Bux70.pdf
(p. 23, in Dutch) http://www.jan-peter-frahm.de/Archive/Archive%2037.pdf
(p. 7, in German)