You should not be afraid to separate the two plants, the Moon valley will probably do fine. Pileas grow almost like weeds! About cleaning the leaves: I don't think the plant would mind a quick shower, if allowed to dry immediately afterwards. Maybe you could pat it with a towel? The reasons why plants with hairy or crinkled leaves like some types of pilea, and african violets etc. don't like water on their leaves, can be 1. fungus might have good growing conditions when the leaves stay wet, and 2. the sun might burn the leaves, as the drops work like a magnifying glass. You could also try with a moist cloth, and wipe off the leaves gently.

About the browning and crinkling of the fittonia leaves - I would guess it gets a little dried out because the bigger plant with the more solid roots take over and helps itself with most of the water. I think it will be better if you separate the two.

Thank you so much for your response! That was exactly the information I needed! I definitely think the Fittonia will be happier and healthier if I move it to it's own space - I'll be working on that very soon! & I'm also glad there's no major harm in washing the leaves as long as they're allowed to dry out in good time. Thank you again! I appreciate it & I know my plants do too! :)

That looks like Coleus to me. Moon Vallir Pillea are usually all green and have smaller leaves than that. I have seen a Pillea similar to that though. I would definitely put the Fittinia in a separate pot.