Just my own follow-up: an insect-savvy person thought this is probably a Plant Hopper (Derbid family). Not necessarily a pest, but not going to help in my fight against the Harlequins and MBB.

Here is a link that might be useful: Bug guide

Thanks for posting that follow-up, I didn't know where to start with the ID because I couldn't tell if there were 2 or 4 wings and any antennae were not obvious; the body looked aphid-like but the wing posture didn't fit.

It didn't look like an insect with either mouth or rear-end parts that would help control your other bugs.

Sadly, no, these bugs apparently won't be of any help in combating the dreaded Harlequin Bugs on our Brussels Sprouts. I have collected some HB eggs and watching to see if they hatch into Harlequins or (hopefully) tiny parasitic wasps. Meanwhile, we are hand-picking the perps.