I'm not sure how much heat (or lack thereof) a 250 generates. Maybe if you bring it lower it will help to evaporate some of the moisture in the rockwool. You definitely don't want the light right on top of the seedlings because you will burn them. Use your best judgement. Once they get 2 or 3 sets of their real leaves you can get the light pretty close. General rule is put your hand under the light at the same level as the tops of your plants. Once your hand feels pretty warm from the light, back the light up an inch or two. Seems to work good for me and I've never burnt em.