Hi @gourabkumar,
Thanks for opening this post.
Well, you can use any Prometheus query to measure the Memory. For e.g. I found this link useful while testing to answer on this post.
This is how it looked like when using the Stat visualization;
Since I have installed node_exporter on 2 machines that is why it give statistics for 2 machines but should also work fine on a single node machine.
Then to create an alert I made a small screencast as it can help you to create of your own. In my case, I set the alert when it is above 30%
To further polish the template e.g. in the slack you can follow the following post as how to improve it.
I hope this helps.