Keeps growing and blocking up my overflow

[MelnSteveC]

Anyone have any idea what this is?

I’m not sure if I should save it and put it in the sump or keep throwing the stuff away.

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[Will Milberger]

Sump it.
When a problem comes along, you must sump it.

 

[Reef of Fillory]

Looks like Ulva [Sea Lettuce]