I think your alkalinity is fine and you should turn the light up to 60% and keep increasing it gradually. If the max output of 55watts is correct for that light, that means you’re looking at about 350-400 PAR at the most from the distance of about 14” is what I guess. Now let’s say it’s 350 and you’re turned down to 40%, so that means your PAR would be about 140 max but it’s probably closer to 125-130 which is not enough light for an Acropora that looks like it should be blue or purple.