If you have the
performance_schema enabled, then this will consume roughly 500Mb of RAM, then in addition to that you need to take into account your buffer pool size settings (RDS defaults these to 3/4 of the RAM).
Adding these two together can push the allocated ram size over 1Gb quite quickly - note that you have 182Mb of swap used too.
The 24.5Mb of free RAM is probably the OS file cache, which can be discarded by the OS should other RAM demands become too much.
Try turning off the
performance_schema in the RDS parameter group and see if that makes any difference.