Your tank inhabitants think that you shouldn't move the rock. It is really stressful for everything in the tank. Aside from that I like the way your rock looks hair algae aside, but if you don't like it you should change it up until you do.
Adding any fine sand will typically end up in a day or two of a sandstorm while everything settles. You might want to get a bucket of tank water with a small powerhead and move any loose frags to it until the dust settles in the main tank. If you can't do that, then be ready with a turkey baster to siphon/blow the sand off the corals, so they don't get starved for light.
If you think you have no room for corals, then you need to look at some pictures of corals in the wild. They'll grow on just about every surface available. You just might have to be creative on how you get them to hold on.
I'm personally not a fan of crabs. They are omnivorous and have a tendency to eat things you might want to keep. That tendency rises the smaller tank is. Snails do just as well without the dangers imo.
