Measure the depth and width of the vertical pond wall to get an estimate of how much window screening you will need to cover the wall.
Cut several sections of window screening to drape over the edge of the pond and cover the pond liner. The fine mesh of the window screen serves as an effective growing medium for aquatic moss.
Tie at least two pieces of fishing line to the top side of the window screening. Anchor the fishing line under rocks or driftwood around the edge of the pond to hold the window screening in place.
Collect aquatic moss from rocks, logs or substrate from rivers, creeks or ponds. Transport the moss back to your garden pond in a plastic bucket with a lid.
Pull the window screening out of the water and spread the moss over the screening by hand. Smother the moss into the small openings in the window screening. It is okay if the moss breaks apart. Small spores that attach to the window screen will quickly develop into a thick wall of moss that hides the pond liner.
Gently lower the window screening into the pond to cover the vertical wall. The moss will develop and spread over the window screen within 2 to 4 weeks.