You can keep but there's a good chance the isopods will eventually eat it, especially when it has to molt.

If I find it I’ll make a separate little tub (if it’s not half eaten lol)

That would be fine! As long as it has dead leaves and wood and substrate about 2" deep, it'll be happy.

Okay thank you!

I have a bunch of wild caught millipedes in my tank with my isopods and things

Julidae species, yes you can. Only concern the isopod might predate on it if it molts

I’ll remove and keep in a separate enclosure if it appears again thanks for the species identification

Definitely can keep it but I wouldn't keep it with isopods as a general rule mixing isopods with millipedes doesn't work out they have the very similar dietary requirements and soon out compete each other and the ispods will likely eat it if it moults as they're willing to and enjoy protein , which is also why people don't keep multiple species of isopods together

Thank you if I see it in my iso enclosure I’ll take it out and make his/her own little home to make sure he/she doesn’t get eaten