Coraline ((vs some of the other nuisance algaes like green hair algae) is neither good nor bad. Many people encourage it by adding a rock covered in the purple. Most people find that it adds a bit of color but it can take over the rock work so that all the rocks are purple. It does not add or detract from the quality of the tank.
Any of the urchins can dislodge rocks while they are feeding.
Most shrimp (but not the more delicate like the blood shrimp) can be placed into a tank after the initial cycle (when the ammonia and nitrite have spiked and then returned to zero). This is generally at about one month of an active cycle. If you start with well cycled rock (like rock from an existing system), the initial cycle can be as little as two weeks. If you will only have shrimp in the system , you will need to keep them fed since there won't be scraps from other animals.
Any of the urchins can dislodge rocks while they are feeding.
Most shrimp (but not the more delicate like the blood shrimp) can be placed into a tank after the initial cycle (when the ammonia and nitrite have spiked and then returned to zero). This is generally at about one month of an active cycle. If you start with well cycled rock (like rock from an existing system), the initial cycle can be as little as two weeks. If you will only have shrimp in the system , you will need to keep them fed since there won't be scraps from other animals.