Looks like coincident surfaces to me:
> plane { y, -10
...
> plane {y, -10
These two planes occupy the same exact space. POV-Ray behaves strangely in
this case (it's in the documentation). Try changing the non-transparent one
to
plane {y,-10.001
- Slime
[ http://www.slimeland.com/ ]
[ http://www.slimeland.com/images/ ]