I was hoping that you’d notice that before exporting :x
Anyway, notice that one tentacle is darker than the others:
This means that it’s facing the other way. When a face is facing the wrong way, it isn’t rendered in Roblox. To fix this, select that tentacle and “Invert Normals”:
You should probably enable “Backface Culling” in the future to avoid problems. This makes it so that faces don’t render when your camera is looking “behind” them (instead of just being darker).