looks like critters...but whether they are causing the problem, or simply free-loading on a sick plant, I can't tell from here.

7 minutes twice a week isn't going to be enough to drench the root system of a yucca, even a young one - so it's possible that it's not thriving because it has too much leaf for the amount of feeder roots its managed to produce.

if the plant pulls right back out of the ground (ie still in the shape of the pot it sat in) , I'd take it back to Home Despot and claim 'rootbount, with failure to thrive'... if it puts up any resistance, it's at least trying - then you can start drenching the hole with water once a week, and add a bit of rooting hormone (or water that willow branches have been sitting in...) to the water to try and stimulate root growth.

you can clean the white stuff off with rubbing alcohol and a cotton ball, or by hosing the plant down vigorously, and then spraying it after each sprinkling with horticultural soap, water, and a crushed garlic clove.