Cup & Saucer Primrose marginal?
 

I have a low growing rapidly spreading marginal in my pond labeled "Cup
& Saucer Primrose." It has a dense root mass and tends to spread out of
the pot. I assumed it was Ludwigia but I can't find any that are this
low. It looks a lot like my Myosotis scorpiodes (water Forget-Me-Nots)
except the leaves are a different shape. Any ideas on what this might
be.