First of all, 40 watts of light on a 45-gallon tank is weak indeed. If you don't plan to keep corals, then I wouldn't worry about it too much. This doesn't mean that you can't have coraline or anything like that, but don't expect such growth to occurr very quickly. Upgrading your lights might be in order, but it's not absolutely necessary.
Also, if you want another solution to your problem, I would recommend seeding your tank with a rock that does have a good amount of growth on it. I would find a fairly small rock packed with coraline (from a store or a fellow hobbiest) and stick it right in the middle of your rock or in an area of high flow to try to disperse the growth onto your existing rock.
92-gallon corner tank, 100lbs of LR
, 140lbs of sand, 250watt 10,000K MH
, 110watts of actinic PCs, Mag 7 return, custom refugium, AquaC EV180 w/ Mag 5
Female lyretail anthias, eyelash blenny, tomato clown, saddleback clown, firefish goby, 2 sand-sifting stars