The photo is unfortunately too small for us to enlarge to see enough detail, but the damage appears to be caused by either mites or thrips. We don't tend to see Crimson Clover grown in a greenhouse (they tend to be sown outside as cover crops), so are not accustomed to pests bothering them this time of year. In a greenhouse, both mites and thrips can be quite common plant pests, in part due to the drier air (this time of year) and lack of beneficial insect or mite predators keeping populations low. Both pests can be suppressed with repeated treatments (follow product label instructions and make sure it's legal to use in a greenhouse setting in the directions for use) of either horticultural oil or insecticidal soap. Sometimes additional leaf "burn" damage can occur after using oil or soap on leaves with heavy insect/mite feeding, but that's unavoidable and any such leaves can be clipped off. As long as the roots remain healthy, new replacement growth should appear sooner or later.

Miri