Hi there. I just took pictures to post in another topic but noticed something while doing so. Some of my pots have a grainy substance similar to a crumble on top. I bottom watered a couple days ago, and I use purified water. I am using Miracle-Gro seed starting and germinating mix over top of an organic base. My greenhouse is indoors in a south facing window, and I also zipper the door shut. Thank you in advance. I am posting a picture for reference. This is the first time this has happened and they were planted approximately 3 weeks ago.