I am not completely sure what exactly you mean by "How do I change the dock icon to an 'X' for close in the slidingpane" and thus I will provide you with some directions for the two possible scenarios understood from your explanations.
1) If you want to have the standard close functionality but not to have the docking functionality you should simply set EnableDock="false" as in the second sliding pane in the demo below:
2) If you want to keep the docking functionality but only to change the appearance of the icon, you should override the default background image with CSS and set yours instead e.g as shown below:
background-image: url(myImage) !important;
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