Looks like it might be a Sponge, possibly of the Tunicates family which are also known as Sea Squirts. They are non-harmful and actually beneficial due to their filter feeding attributes. They may appear in matured systems over time then suddenly vanish, which is why many aquarists find it hard to maintain a colony over an extended period of time. They come in a wide variety of colors and shapes so it can get fairly hard pinpoint exactly what species it is. IME, I have never seen any white Aiptasia or Majano that are clustered together like that before.

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