Knitting

This looks like you purled that first row with the black yarn instead of knit it.

If you feel brave you can ladder down and use a crochet hook to reknit that singular column of stitches. It is relatively easy to do but very intimidating at first, especially with colorwork like this.

I really like tutorials by Roxanne Richardson, who I believe is a master knitter. Here is how she explains the ladder down method.

Otherwise you already know ... Frog it.

You could take a length of yarn (maybe 5 inches?) in a matching color, string it through the dropped stitch so it doesn't fall down further, then weave in the ends.

This would disrupt the pattern, but it's an option if it's somewhere not too visible and I didn't want to redo a lot of work (say, I'd noticed after binding off or something)

It looks dropped to me. Can you chain down with a crochet hook?

Agreed on unraveling the ladder down and using a crochet hook to fix. If you haven’t done this before, I highly recommend making a practice swatch and doing it there a few times first.