Any idea?

[Back where it all began]

Hi, I have seen this before in this same place. It seems to be related to a hole I need the sand just behind it. These are not solid and breakup easily when touched. When disturbed, they produce a “cigarette ash” look. I broke them up several days ago and they just appeared within the last several hours.

 

[Flippers4pups]

Could be detritus from lack of flow.

 

[vetteguy53081]

+1 on detrious.
Siphon up and direct flow so it bounces off glass causing a gyre effect (circular turbulence/movement)

 

[KrisReef]

looks like something is pooping a lot of sand.

 

[P-Dub]

Looks like lugworm, acorn worm, or cucumber castings of some sort to me.

 

[LesPoissons]

My fighting conch made similar casings. Do you have a slug/cuc/conch in there?

 

[Back where it all began]

I thought about a cucumber. I have three that I know of and haven’t seen two for a long time. Cucks make sense as the formations seem to perfect to be dentritus.

 

[P-Dub]

Mystery solved.