Odd that it's blue, must be something in the solder itself.
I'm pretty sure that means a leak (alternatively I'm trying to imagine how some kind low-grade current leakage would cause this only at a joint, and I can't think of any way).
So, while not critical in the moment, this isn't good. My concern is there are multiple bad solder jobs in one spot - every joint in this picture is leaking. How much did this plumber suck, or is copper the wrong material to use in your area because of minerals in the water (while I much prefer copper, there are some areas where the water is corrosive to it).
Fortunately it's not hard to fix, cut everything back, put in new connectors.
I suspect part of the original issue was so many joints that close together. You get one done, and when soldering the next one the heat weakens an existing joint. I'm sure there are guidelines for joint spacing with copper just for this reason (I'm not a plumber, but have done copper work under a plumber's guidance).