These things are growing pretty quickly in my 20 gallon reef tank (10 years old and very stable). What are they? If they're not good I'll need advice on how to get rid of them.
There are products, Joe's Juice is very common. Or you can try boiling water, lemon juice, or kalkwasser paste out of a syringe. Get them now, or they will take over!
Natural options are hit and miss. Peppermint Shrimp are one...if you get the right kind.
peppermint shrimp are nice, if you can get the right kind they will eat it for you but if you get the wrong kind they are very interesting looking and active tankmates but do et rid of the aiptasia, if you try to get it out yourself make sure you get everything down to the roots out or they will come back
don't squueze or smash them their oils will spread throughout the tank and you will have a huge mess. use kalk and make a pultice and squirt about 1cc directly on it,turn off ur skimmer. don't freak out and think it's over because we all wind up with appy's now and then. if you want you can buy aptaisia control by blue life for about 10-15 bucks if you don't have kalk.
Ive used the Lemon Juice method seems to work about 80% of the time. My girlfriend just added to peppermint shrimps to her tank and she got lucky and ate all of hers. So as they say its hit/miss but nothing to panic over just take some time aside to get them under control.
Other Marine Life 
What are these AND are they bad? 
fyi 
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