What is this?

[klavmaister]

plague or useful animal?

 

[IKD]

plague or useful animal?

Looks like a worm or the proboscis of a snail/mollusk

 

[klavmaister]

hmm, started with dead rock and everything gets dipped. started with live sand then, hope that it does no harm in the aquarium. it's not a snail, see several in the same place

 

[ISpeakForTheSeas]

It may be a peanut worm or similar - beneficial/harmless if so.