How to notify Elsevier or Clarivate when then they are indexing predatory or very low quality journals?
First, Elsevier or Clarivate Analytics already monitor things like dramatic increases in the number of articles published. It's why excessive self-citation can and has lead to journals getting delisted.

However you can still write to them to highlight potential problems. If you want to do this, the best way is to use their "contact us" forms on their website. Here's Scopus's page on it, and here's Web of Science's. Be prepared for the exchange to go like any other investigation - you may be asked to provide proof.