Discovered last weekend, the stenciled work shows a forlorn boy holding a can of red paint next to the words “I remember when all this was trees.” But by Tuesday, artists from the 555 Nonprofit Gallery and Studios, a feisty grassroots group, had excavated the 7-by-8-foot, 1,500-pound cinder block wall with a masonry saw and forklift and moved the piece to their grounds near the foot of the Ambassador Bridge in southwest Detroit.