Unknown animal in tank

[Reefncouple]

We have some sort of animal in the tank. It has built itself a sand pile, and keeps spitting dust out of the top. We have not seen it but every day the pile gets bigger and bigger. Any ideas on what it might be? It is 100% a hitchhiker that has gotten too big.

 

[Subsea]

I see this is your first post. Welcome to r2r.

Where did your live rock come from?

How long has tank been set up?

PS: Most likely a mantis shrimp

 

[Tripod1404]

You could try taking a look at the tank in the middle of the night, ideally with a red flashlight.