I got one of them 'Grow it' kits to start with because i've never done it before. They came with the straw-esque pot things and the 'hard soil' that expands when you add water. I planted them almost 2 weeks ago and the only thing that's happened in that what can only be described as mould fluff has formed on top of the soil. I assume this isn't supposed to happen. How can i stop it happening in the future?