It likely is just a catkin. I believe Swedish Aspen is a male cultivar, however, so you won't get any of the fluffy seeds that follow afterward. I don't think it will be harmful to the plant, though mine hasn't bloomed yet (unless I've just missed it somehow). I think in general they are pretty tough plants, though some say, being in the poplar family, they are prone to problems. I think good care, regular watering (if drought) helps to ward off problems.

Glen

Yes, your aspens are engaging in casual sex. The poplar groups are all dioecious -- male and female flowers on separate plants -- I think they are largely wind pollenated, although honey bees will go after poplar and willow pollen as one of the first available bee foods in spring.

Blooming means they are healthy. Expect die off on branches that are in the shade a lot. The dead branches will fall off naturally after a year or two.