Take some extra yarn and close up that loose part as best you can taking care to get any loose stitches so they don't unravel. Then if you want to add a crochet border go for it.
I tend to weave in the end more than some people say you need to. Here's how I do it if you want to see -
For a hat knit in the round I turn the hat inside out and take a yarn needle and weave it across the stitches till I get to the knit part of the rib. Then I work the needle up the knit stitches like in this photo. Then at the spot above the ribbing I weave it in like they do in this link
for about 3 stitches. Overkill? Maybe, but I've never had a hat come undone. BTW..that edge looks like it's loose and lower..it's not. I don't know why it looks that way in the picture.