Shimmerleaf are plants that generate around Silverwood Trees in groups of 0-5. They drop Quicksilver. However, you can only find them around trees created during world generation. Any Silverwood Tree you plant and grow yourself will not generate Shimmerleaf.
To gather Shimmerleaf itself instead of Quicksilver, use a pair of Shears.
"The strange, glowing flowers only grow in the shadow of Silverwood Trees. Instead of nectar, these flowers produce quicksilver."