I'm trying to open a .ply file (point cloud in ArcGIS Desktop and ArcGIS Pro), but I can't.
How can I convert the .ply file to .las, or open .ply in ArcGIS Desktop or ArcGIS Pro?
You can convert the .ply files (aka Polygon File Format) to .las with other software, and then, import it in ArcGIS.
LAStools: supports conversion of ASCII .ply files (treats it as text file) with
las2las tool (see here).