The hashtag symbol appears when the unit is in a pre-heat cylce on start.
It can also appear at random times of the day, this is normal during the winter months as it is signifying the outdoor unit is in a defrost mode. This lasts between 3-6 minutes.
If the symbol is showing for more than 10 minutes then try setting the temperature lower to ease the pressure of the outdoor unit .
If the symbol continues to show then the system is low on refrigerant and needs an engineer to check for leaks and top up the gas.