How clear cached data for all apps on HTC phone running Marshmallow

Yes, this is safe. This will not remove anything you need. And, yes: this works the same way on most HTC phones running Marshmallow including One M9.

I would NOT do this too often myself, as it would just lead to things being reloaded (that’s how a cache works). So, if you are having serious issues with apps: this is one of the things you do/try.

Settings > Storage & USB > Phone storage > Cached data, press “Ok” to clear cache data.

What is cache?

Simplified: A cache is simply a place for temporary storage of data, meant to speed up access to that data.