Hi,
Java fern and Java moss are very different tho they are from the same family. If you wish to turn your budha head into a waterfall then you can use Java Moss as it can grow under water and above water as long as humity is very high. Java moss is a moss with long stems but can be pruned back to give a neater look.
Try to use RO or rainwater at the beginning as most moss will grow better - also this will stop any buildup on your tank glass and fogger. If you decided to use RO water - then before you place in your toads just add some rainwater.
Basically - most of the plants suitable for vivariums do not grow very well with tap water.