I found the same problem in one of my projects: Reference Hardware for Indexers with ES is 16 GM of RAM and I had 16 GB, but the health check is done on available memory that's a little less than 16 GB (in my case 15.88, in your case 15.66).
You have three solutions:
disabling the health check (I'd avoid!),
adding 1 GB of RAM ( the best if possible),
modify the health check search (the last choice) reducing the threeshold to 15.50.
In my project I had a physical Indexer so it wasn't possible to add RAM, so I modified the health check search.