What is this that has invaded and destroyed my erysimum...?

Does anyone know what this plant / moss is that has completely invaded my erysimum and weakened them to the point that they pretty much fell out of the ground?
The erysimum were fairly young plants but well established and over wintered well.....


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Looks a little like one of the saxifrages such as Saxifragia hypnoides but it shouldn't have affected your erysimum. Erysimum are short-lived perennials and it is common for them to suddenly die off for no apparent reason.