I did that! Filled a 65 breeder peninsula tank past the top with rock. Put a mangrove in a natural hole in the rock. At that time, lighting options were limited for me and the mangrove grew to close to the light and heat killed it.
Cool concept if you can work the kinks out. Maybe don’t put light directly over the mangrove
i didn’t do fiddler crabs but remember my scarlet hermits chillin on the rock above the waterline