Is this a hitchhiker or algae?

[fish_freak_jess]

Found a couple of these in my saltwater tank and can’t figure out what it is. They’re on the sand bed.

 

[SaltySqueeble]

Looks like a hair ball. Have any cats? Lol maybe a sponge…?

 

[SaltySqueeble]

Actually
Looks more like hydroids

 

[ISpeakForTheSeas]

Found a couple of these in my saltwater tank and can’t figure out what it is. They’re on the sand bed.

Looks like it may be a very small Tethya sp. sponge such as T. gracilis or similar:

Looks like it may be Tethya gracilis - it should be a harmless filter feeder.

They're not super well known (they were discovered first in a German aquarium), but the study I've found describing them was very informative (linked below).

They grow up to ~1.5cm (though they can contract to be ~80% of their max body size); they can slowly move/"walk" - reportedly, they can move several centimeters a day (especially when smaller), and they do this to remove/shake off sediment that gets on them; and they reproduce by budding (so they'll drop pieces that grow into new sponges).

https://www.researchgate.net/public...hya_Porifera_Demospongiae_from_German_Aquaria