I have trees closer without issue so far for 10 years but don't know anything about box elder.

Id get rid of it now - before it costs 1000+ to remove! Those are fast growing trees and not particularly beautiful or valuable. I dont know about roots, but my neighbor has one thats about 10 feet from my pond and besides a ton of leaves in the fall, it drops all this horrible yellow pollen junk in the spring, and flying helicopter seeds in the summer. All this junk drops in your pond and then you have to clean it out!

It might be nice shade if it was further away - but where it is, tthe volume of junk dropping into the pond would be too much of a hassle . Shrubs + maybe a few shorter decorative trees placed a little further away would be nice - you want some greenery around it.

Have the same problem, neighbor's tree. Ponds and tree's don't go together!