Dose any one know what this is

[Joon]

 

[Cment]

Looks like a ball anemone. If it is its Harmless.

 

[Sabellafella]

Pseudocorynactis, imo there harmless, just very very sticky as they can sometime eat microsnails nd amphipods.

 

[Joon]

How big do they get and if I want to keep it should I feed it

 

[Lionfish Lair]

They stay about that size. They tend to disappear after awhile, but feeding them would help keep them around longer.