saltwater fish tank

First time posting and might need some help to figure out how to upload the video. 😅😭 I have never seen my tang do this before. What exactly is going on here? Is this flashing, territorial display, just my tang being ridiculous? He will swim around the tank looking for food and then swim back...

I have a 10 gallon reef tank that currently has a clown fish, a candy cane coral, and a Xenia. I was looking at getting a fire and ice zoa and a star fish I found a Ehinaster spinulosus or orange starfish and was wondering if it could be in my 10 gallon.

I am new to saltwater aquariums and I have a 6 gallon tank I have done my research but I don't now one thing. In my 6 gallon tank can I have a pair of spotted porcelain anemone crab and 2 pom pom crabs? If not what will I have to turn down Out the the creatures I have just mentioned.