that is a pic of apstasia. they are bad, they grow very rapidly and need to go. my fav. way to do so is to go to the vets and get a small needle and syringe, fill it with 100% concentrated lemon juice and inject them with a little of it. this will start to burn them from the inside out and kill them. it works for all sizes but its much easier to kill them when there larger. If you have a lot of them only do a small amount at a time so you dont disturb your tank a lot.+
What size tank is that, and what are you going to do with it, coral and fish or only fish?? what kind of filter and skimmer are you using, and what type of lighting are you going with?? give us some more info and of course keep the pics coming.