are you using anything else in your tank? soil, MTS, or just straight flourite?
Maybe there were some carbonates that recrystallized and cemented the flourite toghether.
What about the whitish substance, could you tell anything about it, whether it was hard/soft, if it changed on drying out?
may be some bacterial/fungal biofilm or something, if it's a new take, it might go away in a bit after things get settled, if it's established, maybe there was some nutrients in that area (dead plant roots/rhizomes, dead snails, etc.?)
just guessing mostly.