What the...?

I have a small gardening mystery, a volunteer flowering plant that’s popped up in two planter boxes out on my deck.

I have four planter boxes on my deck railings and a round container on a plant stand. I used to put annuals in them, but over the years I noticed that moss was self-planting and decided to let it stay. Eventually I decided to let nature take its course, so that now the three long boxes have long-ago-planted sedum (three varieties) that have spread to fill them over the moss, the short box and the round planter have mostly moss with a few stray other plants that somehow found a home – a clump of strappy grass stems, a bit of fern in the round and a bit of evergreen in the short – and a tiny little shamrock-looking plant pokes its head up here and there in small clusters. It’s basically a sprinkling of random weeds among the sedum and moss, but I like it. Plus, everything survives the winter, no matter how bitter, without any special care, ditto for harsh dry summers.

Now, though, I have a new arrival, a bizarre-looking stem with a cluster of pink starry flowers on top. What on earth could it be?

Update: Found them! They're sedum flowers. Apparently one of the varieties I have will decide to fling up a stem of blossoms now and then.

Update two: But wait! These may not be sedum flowers after all -- further Googling leads me to believe they're hen and chick (sempervivum), another succulent plant I have a few of scattered among the boxes. Alas, once the blooming is over, that plant, its purpose fulfilled, will die. More: http://www.youngs-garden.com/blog/hens-and-chicks-flower/