Unfortunately there is no property that specifies a different alt text for the icon of the sliding zone's button. Still you could work around this limitation by simply defining the Title for the RadSlidingPane and subsequently hide the text only, thus leaving just the icon with the Title value as an alternative text. For the later thing you will need the following CSS:
Hope this helps.
P.S. Could you, please, try not to post RadSplitter issues in the RadSlider forum as the two controls are very different and have nothing in common. The RadSlidingPane is part of the RadSplitter, whereas the RadSlider is a standalone control with purely different functionality.
the Telerik team