I have a walking mangrove in a large pot filled with water and coconut husks. Got it at the local nursery. The mangrove roots are shaped in such a way as to form a sort of cup, where a fist sized chunk of moss is growing. Growing from the moss is a small group of (I think) begonias.

When I got it about 3 weeks ago, there were several distinct begonia stalks with flowers. Slowly but surely the flowers and leaves dropped off all but one. Now, I did notice a white mold spreading on the leaves, which is what I believe killed off the begonias.

So I'm left with one single begonia, with one green leaf (mold free) growing out of the moss clump.

My question is about watering. The guy at the nursery said to be sure and keep the moss wet. I was spraying the moss every day, keeping it moist all the time. I'm just wondering if that's too much, or maybe even not enough? Hard to say weather the mold on the leaves killed them or inefficient watering. Should I ever let that clump of moss get dry? It gets direct evening sun for a few hours a day, but it's hot enough to dry it out. Mold issues aside, I really don't want to lose the begonias because I don't know if I'll ever get anything else to propagate in the moss. The one that's left seems to be barely clinging to life. No idea how the grower did it.

Also, about 50% of the moss clump is starting to turn brown. Is that stuff dead now? Is that a sign I've kept it too wet?

On a side note: Should I ever dump out all the water from the pot and replace it with fresh? I was wondering if it's like a fish tank, where you need some bio-processes happening in the water to maintain the ecosystem. But won't all the stuff in there eventually start to rot or become toxic to the mangrove?

Thanks for the help!