Need info on grass-like plant

[Turin Turambar]

I need a name of grass-like water plant. It covers the rocks and the bottom looks like a green carpet.
Also - is this plant a pest or can be controlled?

Thanks in advance.

 

[Gomer]

do you mean Java moss, riccia, dwarf sag, chain sword, clover or baby tears?

All of these can carpet the floor.

Java moss will grow and stick to about anything..so I think that might be what you mean. ...and in that sense, it can be a blessing or a plague.