After idling for about an hour a little over a stack of slimes had accumulated. That seemed like a very low amount. Simply digging out the platforms and being in the area had yielded several stacks of slime already.

Turns out the wall had turned into stone. It's the stone block I'm looking at in the screenshot below.
Name:  2016-03-30_16.09.35.png
Views: 261
Size:  212.1 KB

I've replaced it with a mossy cobblestone wall. Let's hope this one doesn't get converted into stone :B
Name:  2016-03-30_16.15.43.png
Views: 242
Size:  218.0 KB

Any thoughts on how this might have happened?