Help. Mercuiser D4.2L / Powder on rocker cover

[boatcomputers]

Hi,

I've jut noticed some powder substance on my rocker cover of a Mercruiser D4.2L. It doesn't appear to be salt. Engine seems to be performing fine. Any ideas what it could be, please see the photo below. Any help appreciated.

Thanks.

 

[volvopaul]

Alloy corrosion.

 

[boatcomputers]

Thanks. What would cause this and is it a problem?