White shape on glass

[DraggingTail]

Ideas on what this is?

 

[Holy_makerel]

Looks like an asterna starfish to me

 

[DeniseAndy]

Looks like an astrina starfish. They have a new name now, but I cannot remember the new term. They can be pests, so I would pull as you see. Some are fine. They multiply fast.

 

[cristata.reef]

Now referred to as Aquilonastra. This one is likely an Aquilonastra conandae