Tentacles on the sand bed and rocks only at night

[lost66]

I like to watch my tank at night. Using a phone flashlight I can see the live I don't see during the day. I noticed whole of my tank is covered by these weird tentacles. I am unable to see the "beginning" of them, no shell etc. It is in the sand and on the rocks. Anyone knows what is this thing?

Shared album from piotr 📸

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[Hincapiej4]

Filter feeders, i can't remember their name though.