Fiddler crabs are semi terrestrial crabs. Meaning that they need both land and water within their habitat to thrive. They also require brackish water, meaning that you'll need some marine grade salt, found in the saltwater fish section, to make it. I like Instant Ocean myself for making my marine, brine, and brackish water.
I would recommend that you use sand, play sand is best, for the crab substrate. Its not a must, but fiddlers feel more comfortable when they have a place to burrow into and rest. Just build it at a slope, slowly going down into a pool of filtered brackish water where they can molt. Fiddlers molt in the water, so the pool should be deep enough for them to fully submerge. A 20L is recommended for this because its easier to build the slope of substrate into the brackish pool this way.
Fiddlers are omnivorous, meaning that they will eat both plant and animal matter. You can feed them a variety of veggies and fruits, provided that they are either organic or thoroughly washed, and meats, provided that they are plain and unseasoned. You can also offer algae wafers.
Fiddlers live for about 3 years and don't really breed well in captivity. Their spawned larvae need the currents and availability of microfuana and flora in the ocean to be able to survive.
If the male is turning brown it might be the lack of any of the afore mentioned things.