Found this in my Tank??

[randychuah]

Anyone knows what this is? Is it harmful or its okay to leave it in

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[Michael Hoaster]

Large flatworm? I don't think it will give you any trouble, but I'm not sure.

 

[pagojoe]

It's either a phyllidiid or a flatworm that is a phylidiid mimic. Either way, you probably want to remove it.