clover-like weeds in potting mixture

[laurareeling]

I recently repotted 40+ orchids mainly to get rid of this clover-like weed which had taken over my plants. Today I find it again in one of the most recently repotted plants.

Does anyone know how to get rid of this stuff without repotting? It's pretty insidious and takes over quickly - trying to 'weed' the plant is time consuming and the surface stuff usually breaks off without getting the root anyway....