Q. If prasadam is spiritual why does it decay like normal food does in a
day or so.

A. Prasadam is spiritualized by virtue of its contact with the Supreme Lord. Prabhupada gives the example of iron rod and fire. The iron rod kept in fire acts like fire, but is not fire itself. Prasadam is spiritual in that it has been accepted by Krishna  and thus it has a spiritualizing effect on those who partake of it. Nevertheless, it is still made of material elements and appears to our mundane vision as undergoing material transformations. The same applies to the deity forms of the Lord (archa-vigraha), the body of a pure devotee, the holy dhama (place of pilgrimage) and the sacred scriptures.

An advanced devotee, however  would be able to see the actual spiritual nature of each of these items  and would never consider them according to their material appearance  just as Raghunatha dasa Gosvami was accepting old and spoiled remnants of Jagannath maha prasadam as delightful mercy of Lord Jagannath.