Yes, it is safe to apply granular ammonium sulfate on corn that is already up. As a matter of fact, sidedress applications of ammonium sulfate — often in blends, but also straight — are a very common practice.

Unlike urea, ammonium sulfate is not subject to volatilization on neutral or acidic soil — the optimum soil pH range for corn and most other crops. In other words, ammonium sulfate safely stays on the soil surface until incorporated via rainfall or irrigation.

A few ammonium sulfate granules may get caught in the whorl of plants, which may cause more or less salt injury depending on the rate. This is a localized and transitory effect, though. It is no longer apparent within a matter of days as new leaves come out.