How to clean an old metal newspaper photo engraved plate?

If this is a thin metal sheet with the picture visible as a negative in slanting light, this plate was used on an offset press and the plate is probably aluminum. The white coating on the surface is possibly just an oxidized surface. Try making a paste with cream of tartar and small amount of warm water to form a paste. With a clean cotton rag try rubbing a spot on a non-critical area and rinse.

(There are commercial aluminum cleaning powders but I can't vouch for their abrasiveness.)

In the movie 'To Live and Die in L.A.', Willem Defoe is a counterfeiter and you can see the entire process of creating a offset plate.

My father had a printing business and I remember these plates quite well.

reference: Kipphan, Helmut (2001). Handbook of print media: technologies and production methods (Illustrated ed.). Springer. pp. 130–144. ISBN 3-540-67326-1.