Community of the Android app Boost for Lemmy
That's a Google AdSense thing. The devs don't get to choose individual ads that get shown, and you know that Google sure as hell isn't checking for malvertisements.
You would have to either take it up with Google or pay for the premium version. I've paid for both the Reddit version in the past and currently the Lemmy version, and it's worth supporting the dev for the work they do.
I gotta second the paid option to support the devs. I posted about a crashing issue yesterday and they've already updated the app to correct it. It's a small thing but that kind of effort on a passion project is easily worth a few dollars donation.
Unfortunately it's a cat and mouse game.
Deceptive ads are a violation of the ToS but they take time to be detected.
The paid version of the app is ad-free. Or you could switch to an app like voyager.
I think you can report the ad by clicking on the blue icon on the top right corner of the banner.