(Sorry - I'm addicted to this forum now.) I found three or four strapworts around my local reservoir, flowers all about the same size but with differently coloured sepals. But this plant has a significantly smaller flower, about 0.5mm closed and 1.0mm open. Like the strapwort there are 5 petals, but because they are so small it is difficult to make out any other details (I have included a detail from the full resolution photo as well as the medium resolution full photo).
what a productive reservoir it was! This is a species of Herniaria - Rupture-wort. These don't pose us much ID problems in the UK because we only have 3 spp, 1 of those very rare, another a rare casual - and the third H. glabra is itself Nationally Rare as a native. There are more species in Spain - however your plant is glabrous and most taxa are ciliate or hairier which means that it's likely that your plant is H. glabra. This is probably the most variable and widely distributed species in the genus.