You are fine then. You have 867mb of free memory, with only 126mb actually used.
When you have free, unused memory, Linux will use it to cache objects. When the memory is actually needed to run programs then the cached objects are removed. Your graph in New Relic is showing you memory usage WITH buffers (line 1 in
free -m). Your actual, hard memory usage is represented by line #2.
You're fine, just disregard that graph.