Can anyone tell me what this thing is?

[blocky912]

Hi, I’m new on reef tanks, I recently started 4 months ago, water already cycled have a little bit of diatom and I have four hermit crabs, however I saw something weird growing from the back of my aquarium can anyone tell me what it is?

 

[smitten with ocean life]

its hard to tell in the pic, but looks like maybe aiptasia

 

[Glenner’sreef]

Looks like aptasia. Get rid of it. It will multiply.