White growth on Rocks

Question
I recently noticed white oval shaped spots in my aquarium. It has spread to other rocks and even a hermit crab shell in less than 2 weeks. They are hard, flat and don't come off easily. Any idea on what this might be and more importantly is it harmful to my aquarium?

Answer
sounds like something calcerous, to be honest without a piccy its pretty hard to say what it is.

i personaly have never seen anything oval and white growing in any tank. try asking another expert as i have no idea.

sorry i couldn't be more helpfull