maybe Epilobium sp.
This appears to be the first record of a naturalized population in Pennsylvania. This species is native to Japan and is widely cultivated in the southern US. It is growing on a tufa formation, where the year-round seepage moderates the microclimate, which probably permits this species to grow farther north than it would otherwise.
It is considered invasive farther south, but at this point in time it is not likely to be a problem at this latitude. This population consists of approximately 2 dozen plants.
This Mediterranean species is common in greenhouses around the world, but does not normally become established in the wild in climates like this. I first found this colony in the winter three years ago, so I would call this an established population. There were several square meters of it spread out along 10 meters of an overhanging bank of slag. Most of the plants were looking quite wiped out from the long winter, although some patches were still fairly green. It will be interesting to see if these old lobes die or if they green up again.
Colonies up to 1 inch wide growing on the trunk of a planted Golden Rain Tree. I haven't seen anything like this locally, so it probably came with the tree.
I am not positive about the ID, and would appreciate more opinions.