Hard to tell what it is in that picture. The resolution isn't high enough to really zoom in. Not aphids though.

Not aphids. All aphids that I've seen are either green or black, and they don't fly. Perhaps at another life stage? You could try spraying with neem oil to help protect the leaves.

Stink bugs lay eggs on the bottom of leaves in clusters like that.
Unless you find they're a beneficial insect, I suggest you squish them. May be easier to eliminate the possible beneficials than try to figure out what the culprit is.

Thanks everyone for your feedback. For the moment we have decided to take a wait and see approach. Hopefully they are beneficials and/or just a transient bug.