Minecraft: 1.16.5
Java: AdoptOpenJDK (jdk-11.0.11.9)
Forge: 36.1.23
Tinker's Construct: 3.0.3.152
Mantle: 1.6.97
Seed: -8836811803329303795
Tinker's Construct Floating Island Coordinates: x=-133; z=150; y=-145

On a new world on a fresh install (only Forge and TC+Mantle), Floating Islands generate as vanilla terrain without the "slime-type" stuff.

That's a new clay island. It's intended.

It should be the tinker clay island check in the pond to see if there's a clay

So it is!

Thank you!