Yes, the white stuff is probably perlite from the potting soil. This would not cause the fire as it is a type of light weight rock that helps drainage. The other ingredient in potting soil is peat moss. If the planter is under the eaves and the soil is very dry it is possible for the peat moss to become so dry that it is combustible. A spark can ignite it. This happens in nature when there is a storm and electricity from lightening ignites old stumps. Be sure to stir up the soil in the planter and add hot water to help moisten the peat moss and add some compost to the old planting soil before you plant. Living plants in the planters will be much safer than dry soil. Keep growing, Marianne Binetti