I do believe otos eat that brown algae. But I would not wait for a fish to clean algae off of a slow growing plant. I would attempt to remove it manually, and if you can't rub it off try direct application of Excel (not in excess of the daily dose). This is because algae will permanently damage the leaves and the leaves are replaced so slowly on a java fern. Faster growing plants don't require as aggressive action as worse comes to worse you can dispose of the damaged section of plant knowing it will rapidly grow back.
I think your tank looks great. Some other good low light plants include bacopa, rotala, and crypts. Rotala can get a bit pinkish towards the tips so a good way to have some color other then just green in your plants. Crypts also come in different varieties, the Crypt wendetti's are easy to keep a good bronze crypt would look nice, sort of orangish brown color.
The grey stuff on the moss just looks like a build up of tiny particles of detritus, mine gets it too.
Just give it a gentle shake with a chopstick or something to release the particles into the water column for the filter to deal with.
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