Relatively small root -- shouldn't be a problem to cut it off as close to the root-crown as reasonable. No reason to wait IMO. And don't cover up that root crown.

Maybe I am confused by your description but if the root is growing out laterally and not circling or wrapping around the trunk, why would you think it is girdling?

It's not girdling, but it appears that it might be crossing over the big root next to it. I've taught my guys to play it safe and remove roots like these that don't take a more direct, straight out path from the trunk.

I would wait until fall to cut that root off. Young trees can already have enough stress on them due to having been transplanted and dealing with possible harsh summer weather like many saw last year. No need to add root removal while in full leaf to that.

That'd be my concern too, crossing over the bigger root. It's prb'ly not a big thing, but then neither is cutting off that small root....

Tree roots grow over and on top of each other all the time. Not anything I'd get very excited about.........