algorithmic modeling for Rhino
It does exist...
A very early version of Grasshopper for Mac is available in the RhinoWIP for testing and feedback.
Users of Rhino 5 for Mac have access to the RhinoWIP.
The most recent version of the RhinoWIP contains a command called "Grasshopper".
Please post feedback and report bugs (there are many!) on the Grasshopper for Mac category on Discourse.