Healing Tool leaving white spots

Your screenshot shows a 'dot' below the Masking button which tells me that you have at least one Mask present on this image. This 'dot' is highlighted in red which shows that some auto-detected Masking needs update.

A Mask is applied which uses (say) a 'Background' auto-selection. This selection is made by looking at the picture content. Then the resulting 'background' area receives your adjustment - let's say, darkening.

Now you do some cloning which modifies what the picture content looks like. The mask you currently have is based on an analysis of the image as it looked before the cloning happened, so there is a potential mismatch between the remembered mask, and the image content as it now looks. This usually looks like a sort of shadowing or patchiness.

There is nothing wrong with the cloning itself: you just need to tell LrC to re-do its AI detection of Background, or Sky, or Subject - in respect of this different looking picture content. 

IOW we must choose a working sequence: either [clone] > [mask], or else [mask] > [clone] > [update mask as needed]