Ouch! Sorry about that.. That looks like a classic case of dezincification. Dezincification is what happens when the zinc content is eaten out of the bronze alloy. Look at the pinkish coppery hue working in from the outer edges. That bronze, most likely manganese bronze, should be goldish yellow not a pinkish coppery color..
You want to be careful painting p-struts as the copper paint can accelerate the dezincification process. I would suggest that when you get it fixed you have a marine electrician who knows corrosion come down and check things out. Also check your thru-hulls/seacocks if bronze and rudder shaft. You may want to install a zinc right on the p-strut too.
Here's another example of dezincification.
And a prop suffering from dezincification: