"Is the wrist band that was on Niylah's wrist in the first episode ever going to be explained?"

Does it need to be? She wore it as a symbol of her regard for Skaikru, for their role in destroying the Mountain that took her mother. She talked about her family’s history with Clarke in 301. We saw grounders going through the remains of the dropship in 201 and have elsewhere established that they collect interesting artifacts. Bellamy and co. tossed the wristbands aside like trash in season 1; they were clearly found by grounders since then and have been passed around. I’m not quite sure what needs to be explained.