Java fern or anubias. Lots of varieties of both pretty readily available, so you should be able to find something that works well for you. Both are easy to attach to dw and are easy to trim sections out if they start taking over more than you want. Bonus: you'll then have plants to sell or trade. 
If you want something really small, look for anubia nana petite--leaves can be as small (or smaller in some cases) as a pencil eraser.
If you want something really small, look for anubia nana petite--leaves can be as small (or smaller in some cases) as a pencil eraser.
