That is scale. Not usually a problem with clematis but it can happen.

The easiest way to control is to pick them off. They should pop off with just a fingernail. Or you can use a systemic insecticide listed for scale control.

Hi

Is there a reason why I got it last year and again this year?

Destroyed plants that succumbed last year!!

They may have migrated as juveniles (very different appearance from the adults) from last year's plants and overwintered on this guy.

To be perfectly honest, there is really no good explanation why these kinds of insect pests appear. For example, where do aphids come from? They don't fly and can't 'walk' into a garden and if you or cold weather have eliminated any eggs or haven't brought in anything new that was infected, just what causes them to miraculously appear? I swear it must be something like spontaneous generation :-)

I think they overwinter in the soil or mulch. Just a guess..