maybe the shrimps are molting who knows, they should be coming out soon though. Here is a link to some info on the red mangroves and how to grow them:
ReefCleaners.org - Growing Red Mangroves
You can also get a cheap hang on the back filter and put them in that, it works, but the roots will probably be stunted a bit. I did what you are talking about in a 20 gallon display, worked fine for me. The roots will grow around the rocks, but when they get too large just pull them up and trim them. They grow pretty slowly so it won't take much. You may also want to carpet the bottom of the tank with some chaeto, looks good as a carpet, and works great with mangroves. Good luck to you!