How to fix 'App Cleaner' virus-like pop-up on Mac after RAM upgrade?

about 6 months ago, i got a mac and setup everything. i downloaded an app called app cleaner of some website, and it worked perfectly. fast foward to now, there wasnt enough ram, so i upgraded. all the backup on one tiny tiny sd card. 2 days after setting eerything up, i get the "app cleaner" is damaged and can't be opened. you should move it to the trash. EVERY SINGLE TIME i press any button, it pops up again. just like the cartoon windows viruses. it is so annoying i wanna punch my new computer. i've deleted every trace of it i can find but nothing works, pls help I’m desprate.

[Re-Titled by Moderator]