HELP! Don't know what this coral-looking thing is

[Robert96reef]

My 10g saltwater tank has been up and running for a little under a year now. Started off with live rock and sand then added 2 clowns, a yellow gobby, and a full clean up crew. Recently I noticed this strange tentacle stringy coral-looking thing growing off of one of my live rocks. I don't know exactly what it is. It started growing oit of nowhere. Is this thing harmful to my tank? Or should I let it be?

 

[bif24701]

My 10g saltwater tank has been up and running for a little under a year now. Started off with live rock and sand then added 2 clowns, a yellow gobby, and a full clean up crew. Recently I noticed this strange tentacle stringy coral-looking thing growing off of one of my live rocks. I don't know exactly what it is. It started growing oit of nowhere. Is this thing harmful to my tank? Or should I let it be?

Aiptasia. They can spread and sting coral, don't want them so look for ways to kill them.

 

[Z-man]

Looks like an Aiptasia. For sure the back left is.

 

[FlyinBryan]

+1 aptasia kill it now or you will regret it. They grow like weeds and sting everything.