You wouldn't see a jokullhlaup from the air. Reason being, it's under the glacier and you're above it. Pretty sure that what you'd expect to see would be the area in front of the calving front being swept clear by the flowing water - not what you see here.
What you see here is that the glacier surface looks a bit darker than the surrounding ice cap and sea ice. Far from uncommon, you can see the same thing on many of the Greenland glaciers (and indeed on this one in previous years). Not sure why, could be the snow melts back earlier, could be the surface is rougher, could be something to do with the elevation and the angle of the satellite.