Is This a Sponge or Anything I Should Worry About

[retiredsailor]

the objects are located on the backside of a yellow feather duster I bough from AquaSD back in March. It hasn't moved or caused any. Problems that I am aware of. Please take a look and let me know what you think. Thanks in advance.

 

[143MPCo]

In general most sponges are filter feeders and ok in a reef tank, some become an issue when the start to grow and compete for space with a neighbouring coral.