A lot of these micro ferns seem to put out a long runner like that, then once it's happy, a series of leaves will pop out over the course of a few days. As long as it's growing, staying green, and not dropping any existing leaves, you're good!
What he said.
This is exactly my experience - my vaccinifolia shoot long hairy runners all over the place. When a runner gets happy, boom, leaves pop out all along it. This may not happen for weeks or even months - and then it does. I've also got stretches of long hairy runner that seem like they won't ever get leaves. But there are places further along - newer growth off the same runner - that "got happy" and are covered in leaves. Consistent but not excessive water seems to make them happy. I've got a lot of wet spots, dry spots, and in-between spots all around my vivs due to spotty drip walls, gaps between mist heads, etc. The plants deal with it, winning and losing here and there, as micro-site conditions favor or disfavor them.
I really love this fern. It's super cute, not a rampaging beast, and pretty damn tough actually.