It sure looks like hard water deposits. It is just CaCO3, either from the carbonates precipitating out with heating water, or possibly from certain plants that can extract carbon from the bicarbonates. (Biogenic decalcification)
I don't think you would disturb things too much if you remove it. OTOH, having CaCO3 in the tank is just like people adding crushed coral, it provides a source of buffer in case you are using up KH. It doesn't do any harm to leave it.