(AWS) Mysql memory alert threshold(for monitoring) and innodb_buffer_pool_size
With the buffer_pool set at 75% of RAM, it will grow until it consumes that much of RAM.  But there is a lot of other things in RAM -- the OS, MySQL code, other caches, buffers, etc.  The real goal (for a server running only MySQL) is to consume 100% of RAM, but no more.

The buffer_pool is the biggest item that can be tuned.  75% is a good number (assuming you have at least 4GB of RAM).  Only in extreme cases (or you fiddle with other tunables), will you blow out 100%.

Using more memory than you have is really bad because it leads to "swapping".  But MySQL is not coded to understand that.  It assumes, for example, that the entire buffer_pool can be accessed without any disk I/O.  Disk I/O is a performance killer.

I don't know what AWS means by FreeableMemory, but 66% seems fine.  Even 25% seems fine.  5% would seem dangerously close to swapping, and I would worry.  None of the buffer_pool should be "freeable"; it sounds like the buffer_pool has not yet grown to its stated max.