Sorry, forgot the link :-) https://sourceforge.net/p/geanyhighlightselectedword/code/HEAD/tree/GeanyHig...
It's actually already referenced on the plugins website on the lower left at "Third-Party plugins". https://plugins.geany.org
Also whats the difference to addons "highlight selected word" function and core "mark all"?
I'm not sure. Do you think of the "Mark" button in the search dialog (CTRL-F)? If yes: Difference is, that "Highlight selected word" is much faster to use. No switching between editor window and search window. Simply select the desired word by double clicking or using Shift-ArrowKey. I find it pretty convenient for reading code and identifying where a term (e.g. a variable name) appears.