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The new rock work looks great. It might slow you down a bit at the beginning here to change the sand, but you just saved your self a ton of headaches further down the road. We had crushed coral in two of our tanks and replaced it after we already had live stock in the tank which was not fun.
You can pull them out at night with tweezers if you can grab them. Arrow crabs and dottybacks will eat them, but I would not recommend putting either into that small of a tank. The arrow crab maybe as a loaner, but once they go through your bristle worms they might turn on your snails yet.
Don't know too much about the starfish, but the worms are common bristle worms. They are essentially a free clean up crew as they eat food that is not broken down. For a while they were considered the mortal enemy but now most people will let them be because they are a good part of the cleanup...
Added a bullseye pistol shrimp a little while back and finally got some pics of it. If you are considering a reef lobster, I would recommend one of these guys.
What has the bryopsis on it? If it is on something that can be soaked in vinegar or scrubbed that is how we got rid of some. Whitebird has huge success with raising his mag level up. He has a thread on it in here or I know you are friends with him so give him a ring. He tried a 3 day lights out...
I will get another top down shot, unfortunatley there will be not shots of the clams as we lost those :( My computer is acting up (can't connect to the internet so pic posting has slowed down.
Is it bryopsis algae? That stuff can be really tough to get rid of. After it is in your tank either really high mag levels or scrubbing it out will get rid of it. Good luck. Water changes and nitrate levels do not make a dent in it.
If you have a smaller tank I would be very cautious with a coral banded. We had one in a 20g and he would molt, then eat the smaller shrimp. He went through 5 shrimp like this. Camel backs are not reef safe so as Nick said be careful.
Don't add live rock or live sand to the QT. Add some pvc pipes or other things that are not going to be able to harbor ich and are easy to clean. For filtration you can use a simple HOB filter, will provide filtration and can house the bacteria.