Lemon Balm under attack!

This morning I woke up quite early, so made myself a cup of coffee and went for a wander outside to inspect my plants.

And I had a nasty shock.

My lemon balm plant was in a very sorry state.

I'd always thought of the lemon balm plant as super duper hardy. Yet, it was under attack by some mystery white bugs.

See how the leaves are discoloured?

Another close up.

I found strange white bugs on the underside of the leaves. When I touched them, they flew away.

Help! What on earth are these things and how do I get rid of them? I'm also worried that these bugs are going to infect the neighbouring plant - the mint. Eep.