Floating Green stuff

[gibbanator]

Does anybody know how I can remove this floating green stuff? It looks like the type of water you can change your pool water to but I have no idea. I've been trying to remove it. I go up and down on the floor levels, but I cannot click it in any way. It looks like it may be a floor type, but I can't even lay a floor on it. https://i.gyazo.com/726f1b23665f77ec9026a103f0df6f8b.jpg
https://i.gyazo.com/92b09be024a9674a791fccefd1be425c.jpg