Bunny Head shaped critter

[Pstreet]

I've never had anything like this in my tank. It is not hard like Coraline but it shows up on my aquarium glass. When I remove it from the tank it's soft bodied and will roll up but doesn't smear as easily as red slime. All of them are the same shape: roundish with Bunny ears.

 

[ISpeakForTheSeas]

That's an acoel flatworm of some variety.

 

[MrGisonni]

Flatworm