The typical response to under lighting is for a coral to try and gather as much light as possible. LPS can form a 'trumpet' shape as they reach upward to get closer to the light and/or expand outward their top polyp surfaces.
First signs are shrinking of the flesh, extrusion of the mesenterial filaments and a lack of feeding response. Either partial or total bleaching can follow and the coral may/may not die depending on severity and duration of the event.
Too little light is a lot less detrimental than too much light, but given enough time the coral runs the risk of starvation (especially if not fed food regularly).