White Kimchi problems

Hi! I tried making white kimchi last weekend. Left it out for about 3 days then put it in the fridge. Ate some the day after I made it with bibimbap and it was really good. But after I put it in the fridge, it started turning brown on top. I had pressed down the kimchi to keep it all underneath the liquid, so I don’t know what the problem is. Every day the brown layer seems to go a little deeper. I plan on removing the brown kimchi today and trying what’s underneath that still looks good, but I’d like to know why it turned brown in the first place and how to keep it from happening again. I love white kimchi and would like to keep some made for when I don’t want the spiciness of regular kimchi.

Stacy