My clones do that just about everytime and all I use is the Rockwool cubes to start them. If the roots are visible out of the cube then plant and go, if not then (if it was me) I would mix up 1/4 strength nutes and "dunk" the cubes into the water for about 3-5 seconds. Also if rooted get rid of the dome unless you want a mold problem. to much humidity when rooted can give that overwatered look, been there done that about 2 grows ago and almost lost half the clones b4 we figured it out.