Yes, this is normal. Linux will use any free memory if needed for cache and buffers - it will be reallocated if needed for anything else (i.e. stating a new program, an existing program needing more memory for something, caching more recently accessed information, or the hypervisor requesting some memory back by inflating the local balloon if you VM is setup that way, and so on).
This is all managed automatically, but if you really want to fully clear it yourself you can by running sync; echo 3 > /proc/sys/vm/drop_caches as root. The only reason I can think of for doing that is if you are running performance tests and want a repeatable "from cold" starting point without completely restarting.