Christina,
I would have said it was the planter box if it was older...
Sometimes when wood breaks down that happens..
But, I am wondering about the bottom of the planter box, does it have drain holes??
Sometimes, there may be a wood material *ie; peatmoss* in potting soils which is decomposed hunks and pieces of wood, so it may be just that.
If it doesn't recieve alot of sun and it's a tad wet, it could be the combo of the two..
Is it possible to scoop out the formations?
And see if they keep coming back?
Your
plants are doing okay though?
And keep us posted???
Weezie