taking rock from the beach is as far as i know against the law
here,i don't know about river banks.
anything taken needs a quarantine because of natural beasties and such.
if the moss looking stuff is on the stones in the water,perhaps
it's just an algae which you can proper gate yourself by leaving
stones in a jar of water on the windowsill…other than that if it's
flowing like hair…leave it alone…i almost introduced hair algae to my tank
from the weed from the pond…
and if it's moss on the edge of the water then it will probably die off under water.
this probably has been no help at all..just thought my 2p worth.