That's a scale insect. I've never dealt with them personally but since they have a hard and very protective carapace, I'd be inclined to use a systemic insecticide against them.
That's a question for your hydro shop guy unless someone with more experience using systemics can come along and help- they can be dangerous, and should never be used on a flowering plant, or anything even remotely close to harvest. I've used Avid in the past but allow at LEAST a month for it to work out of my plants.