Looks like a long piece of slime.

[MythN7]

Not sure why the water is doing this.  Maybe cause of the ladders?

https://gyazo.com/70ec93cad41f05f3ecf23b7873e19c91

Reminds me of when I was a kid and after drinking milk seeing how far to the  ground you can get your spit to dangle.

LOL

[GreatGameDota]

They're called boogers, a gift of the jelly water physics. Usually happens with polluted water and a small amount of water.

[Kasuha]

There are two ways how water can fall in ONI - it may be either "waterfall" where the water keeps filling the tiles it goes through, or it may turn to "droplets" in which case the tile keeps its original gas contents and water just goes through without affecting it until it hits something where it materializes again. Yours is the first case, there's enough water going over that edge that the game engine decided it will keep being a spacefilling mass. And the rest is up to the graphics engine where the game renders a tile filled with water as sticking to any surrounding liquid or solid tiles, but draws a surface towards any tiles containing gas.