I think it is a doublefile Viburnum, perhaps a variety called Wabi-Sabi. Could it be one plant that popped through your Juniper in four places ? I would wait until Fall to try to separate the plants. Viburnums are beautiful, but some get quite large. Wabi-Sabi only gets 2-3 feet tall and 4 feet wide, so hopefully that's what you have and you can find a place for it somewhere.

I would rather lean towards Hydrangea paniculata, perhaps a seedling, or, if there pop up 4,

leftovers from some while ago

I seriously doubt that is a hydrangea. White flowered serratas are almost nonexistent. Of the one or two in cultivation, the fertile sepals are of a different shape and the leaves do not match. It is also not common for these to self seed. 50 years of growing hydrangeas and I've only seen it once or twice. Also pretty early in the season for a serrata (or a mac) to be blooming unless in the deep south.

IMO, much more likely to be a viburnum and various species and cultivars and associated flowering times could fit the bill.

Have to say that going by the flowers and leaves I’m in the Hydrangea camp. There are a fair few white serratas. But I do find it odd that these, whether Viburnums or Hydrangeas, could just appear out of nowhere as volunteers.

One that I rooted from a neighbor's plant.

And another that came up as a seedling. I've had quite a few seedlings from my Hydrangeas.

Wow Carol, I am SO jealous! I've had numerous hydrangeas tip root but never a seedling. You are so lucky!