.ply files are not supported in Unity without plugins. You can convert to .obj, .fbx etc by downloading Blender, importing the .ply file, and exporting as whatever you want. The issue is that .ply files use vertex colouring, and .obj files need to use a texture file as a PNG or something.

If you convert to .obj using MeshLab, the colours will only show up in Meshlab (since Unity will not import vertex colours from .obj files since that is not part of the .obj spec, MeshLab just adds it the .obj anyway).

What you should do is export the .ply as an .fbx file using Blender, and then import that into Unity. Then to get the colours to show up, you need to go to your .fbx file, and go to the inspector. Then click "extract materials", and edit the material that pops out (should be called material_001.mat or something). Edit it by changing shader from "Standard" to custom/unlit vertex shader. And then you should get colours to show up in Unity.