I would place the mulch first, then sprinkle the alyssum seeds over it. Alyssum is notoriuous for coming up in cracks, but if there's a chip on top, it can't lift it.
Nell

The whole point of mulch is to keep sunshine away from the ground so that (weed) seeds don't sprout (and, of course, to hold in moisture). Of course, it has the same effect on desirable plants. If you want something light to cover your seeds in the pot, use vermiculite. Otherwise, don't use mulch until you put the plants in the ground or their permanent pots for the summer.