Such plants with unusual leaf shapes (3- or 5-lobed, with a long and narrow middle lobe) have been identified as M. indica on iNaturalist. Typically, leaf shape in Morus is drastically variable within one tree, while in these plants it is much more stable. They deserve further study.
This young plant was found in Olmsted Park along Muddy River, 2.3 km north of M. indica planted on the Arnold Arboretum grounds.
A total of 6 occasions of spontaneously growing plants of this kind within and north of the Arnold Arboretum are presented here; some more can be found on iNaturalist. [map] (yellow = without salicicola photos).