Maybe the glass is blocking some of the UV, but the nem should get used to it. If you prefer to have the cover on the tank, just leave it on and the nem will adjust to the difference in spectrum or intensity.
Brs did a video a while back and said glass covers can block a pretty good amount of light. Can’t remember exactly how much. I had glass covers and they used to get super gross and foggy. Ended up making my own out of this but in white. They sell it in 1/8” and 1/4” I got the 1/8” https://www.bulkreefsupply.com/1-4-black-netting-bulk-reef-supply.html