Hi Emir
the search module is the std Joomla search module with a template overwrite in
templates/flexible_lightmart/flexible/systems/joomla/layouts/mod_search/default.php
the code is
<input type="text" on-focus="if(this.value=='<?php echo JText::_('MOD_SEARCH_LABEL_TEXT'); ?>') this.value='';" on-blur="if(this.value=='') this.value='<?php echo JText::_('MOD_SEARCH_LABEL_TEXT'); ?>';" value="<?php echo JText::_('MOD_SEARCH_LABEL_TEXT'); ?>" size="20" class="FlexibleInputbox" alt="<?php echo JText::_('MOD_SEARCH_LABEL_TEXT'); ?>" name="keyword">
so you should be able to translate it with
MOD_SEARCH_LABEL_TEXT