Yep, have to agree with lupin here.You could try some mollys but be sure to condition them to salt since they are normally kept in fresh all there life.
As for plants you could try some java moss (might not grow) or mangrove trees would look awesome
Fiddlers are not only brackish water, but they are best kept in a 50/50 water/land tank as they need a beach on which to rest out of the water. My crabs almost never entered the water and spent most of their time on the beach.
Then it's a male fiddler crab if it has one claw larger than the other. Not every LFS knows their stuff. Post the pics which is an important step right now.