Whats going on here?

So yesterday i was in the garden enjoying the sun while it lasts when i spotted some ants gathering round a white powdery substance along one of the tops of my plants. Can anyone tell me whats going on.

I have had a look around and i can only find it on one plant, will it spread, should i be worried. It will be much appreciated if anyone could tell me what this is and why these are here. Thanks