Why did federalists target immigrants in the alien and sedition acts

Because alien means unwanted or illegal immigrant

The Alien and Sedition Acts were a set of four laws passed by the Congress of the United States in 1798, in anticipation of a war with France. They made it harder for someone to become a U. S. citizen, legalized the president to jail and deport non-citizens who were considered dangerous or from a hostile country, and criminalized making false statements against the federal government. The federalists claimed that these laws would protect the country during a de facto war against France.

The decrees, made quickly following the XYZ Affair, put limits on foreigners and inhibited the criticism of the press towards the government. These were aimed at French and Irish immigrants, increasing the waiting period to obtain naturalization from five to fourteen years and authorizing the expulsion of foreigners that were considered dangerous.