It took them about 2 1/2 weeks, but the mass of Bryopsis is gone.  The slugs consumed a lot, spending almost all of their time face down in the stuff, but some of the algae may have died due to my inability to keep NO3 and PO4 at measurable levels.  I have been adding KH2PO4 and KNO3, but the plants have been consuming it like crazy.

Here is what the tank looks like now. Rather different from a week or so ago when it was stuffed with fresh algae.

A couple of them were chasing each other.  Courtship?  Will we finally see eggs?