About thready growth from Boston fern
What more do you want?
What is it you want to know? Is the growth common? Natural? Harmful? Meaningless? Well, to answer all your unasked questions, these growths are called runners, or stolons. They are natural, and can be used to start new ferns. Here's more information: https://www.gardeningknowhow.com/houseplants/boston-fern/boston-fern-runners.htm