Based on your question I am going to make the assumption that your ad network uses frames to display the ads and doesn't inject them directly into your DOM.
Based on that assumption is not something that you have any control over and is highly unlikely to cause any issues for you. Google will be able to detect that the advertising is being done through a frame and that the source of the frame is at a different domain to your site and as such should not be classed as cloaking on your site. It is also quite common to use a robots.txt disallow rule for advertising networks actual ads being displayed as those are not organic links and as paid links should not be designed to affect the destination pages ranking in line with the webmaster guidelines.
The other point I will make is that Google, being one of the largest advertising networks in the world as well as one of the major search engine providers in the world, has become extremely good at differentiating between actual content and paid advertising. As paid advertisements do not constitute organic linking Google does not include ad network based links when working out ranking for a given page.
The reason why Google shows it as a "low impact" and not a "no impact" is basically because even the tiniest thing can show up as a low impact even if there is nothing you can do about it. The point of "low impact" is to bring it to your attention before it becomes an issue and give you the chance to change it if it requires it, however in this situation there is nothing really that you can do.