The Dreaded Majano Anemone
The majano anemone presents somewhat of a paradox. Many owners of saltwater aquariums want to kill them off at all costs, yet there are others who would like to purchase one or more if they can find them for sale. The majano anemone has been described as "cute" and also as "terrible". It is in truth a rather attractive small anemone, having fluorescent green tentacles, and in an aquarium where there is not an abundance of nutrients, it may be kept under control. If the environment is rich in nutrients however, you can soon expect a tank full of these anemones. Probably not your goal as a saltwater aquarium keeper.
The Majano Problem - The problem the majano anemone presents is that while by itself it appears to be a rather docile creature, minding its own business, it has a disturbing tendency to divide and conquer. In other words it is extremely invasive. One can have an attractive saltwater aquarium, complete with a coral reef and a majano anemone or two, and before long, everything is covered with the anemone. So, it's little wonder that it is an undesirable creature to have. Not only undesirable, but in the view of many, very difficult to bring under control or get rid of.
Find Something That Eats Them - One can attempt with some success to scrape them off, especially if they are attached to rocks. Or, if only one rock is affected, you could simply remove it from the aquarium and throw it, and the attached anemone, away. It’s usually not that simple though, and one usually has to end up killing the anemone or finding something that will do the job for you. There are a number of creatures you can put in the aquarium that will eat the majano anemone. One has to be careful though, as some of these little helpers will eat other things as well, residents of your aquarium that you would rather not see eaten. Raccoon butterflies and tasseled filefish will feast upon majano's, but even here, one doesn't know for certain what else they might eat. After all they have to eat something. They must be somewhat safe to use however, or they wouldn't be recommended as a solution to the majano problem.
Forceps - One can also try removing the majano mechanically, that is with forceps. This might be the best approach of you have only a few to contend with, but can be an overwhelming task if you've let the situation get out of control and have dozens, if not hundreds, of the 'cute' little fellows to deal with. The majano attaches itself to any surface with a thick leathery foot or pad, and the task before you will be to pry this foot off and not leave any part of it behind in the process. Leave a piece of majano behind, and eventually you will have a new majano in your tank! But in any event mechanical removal is one approach.
Kalk Juice - If you can manage to kill the majano anemone while it is in the tank you don't necessarily have to remove it, although if they are numerous you may want to in the interests of keeping the water clean. Leaving a few dead ones in a large tank isn't going to hurt anything. Other denizens of the tank will probably eat the remains without coming to any harm. We're not talking about a potent poison here that's going to kill off every living thing in the tank, just a substance that the majano anemone can't tolerate. One such substance is kalk juice, which is simply a paste that you put in a syringe and inject or "feed" the anemone, killing it. Kalk juice appears to be about the most effective method around, at least according to those who have tried it.
Joes Juice - Another majano killer is a commercial product called Joes Juice. While there have been a few users who say it is not terribly effective, at least not as effective as is kalk juice, the overwhelming majority of users says it works and works fast. Like kalk juice, Joe’s juice is injected into the anemone with a syringe. Again, whether you wish to leave the remains in the tank or not is something for you to decide.
If you really like the looks of these little creatures, you still should eliminate them from the main aquarium, but may want to keep a few in a smaller aquarium, where they can be kept under control by controlling the amount of nutrients available.