Tiny rust spots from nearby grinders
It wouldn''t hurt simply to state the facts to the yard owner. Ideally in person, phone is OK, e-mail or letter is right out. Just the facts, no accusation or request for redress, no hysteria. "Bob, some guys were grinding a steel hull upwind, and now I''ve got these annoying rust spots all over my new alwgrip." Period. Silence. Pause.
Let the yard owner come back with a response. Who knows what it''ll be? He might recommend a product or technique, or offer to fix it for you, or offer to help you go after the other boat, or tell you to buzz off. One way or the other, that''ll tell you how to proceed next.