Are you noticing a white top layer? Fuzzy or Hard?
This could either be as a result of a salt buildup or a bit of white mold. Luckily, these can be easily resolved by spraying the affected area with some of the hydrogen peroxide from your kit (or some warm water and dish soap).
If the texture is white and fuzzy it is most likely mold. This is as a result of some dirt getting onto the peat moss plugs of your unit.
If the texture is a little harder, this is as a result of salt buildup. This indicates that your plants are getting more nutrients than they need. Its important to ensure your pH levels are within 5.0 - 6.0 and your EC levels are within 1.2 - 1.6 mS/cm. If you wish to purchase a pH and EC meter you can do so from the accessories page on our website.