It looks like it could be this Outdoor Voices top https://www.outdoorvoices.com/products/cloudknit-longsleeve?variant=18152120069View attachment 26100
I love Marine's super simple workout gear. Any idea where to get shirts like that? I know it looks really plain and simple but I just went shopping for workout gear in the city and did not see this kind of loose shirts during the first browse. I guess H&M or so stocks something like this in their nightwear section but the quality is so bad that a few washes will ruin it.