Candytuft is a perennial. Don't think alyssum is. Not for me anyway. I think Debin has it right.
Spirea is a shrub but the blooms on candytuft are very similar and they bloom at the same time of year.. You still have a nice plant.
I think it is candytuft. It bloomed last year, leaves stayed green all winter, and now it is blooming again. Should I move it out of the pot into the ground? I am doing an all white area. I really like it.