It looks more like insects and less like a disease. At this time of they year all of them are out in force. If you can get a magnifying glass or better yet a microscope, it would be good to look on the underside of the leaf and see if you can find any bugs crawling around. If you have a master gardener office near you, call them they will do pest id for you if you bring the sample in. I have a hand loupe, it comes in handy but it does require patience and some skill to get used to using it in order to focus and you have to move all around the leaf until you find something.
My guess is that they are aphids. Although at this time of the year thrips, whiteflies, and mites are also around and you may have more than one problem.
if you have ants around, put out ant bait.
You need very good sanitation. Get rid of the sickest plants and leaves and make sure you get rid of any leaves that drop.
Natural predators would be ladybugs.
If you have to spray, you can use insecticidal soap or neem oil. It will have to be done weekly.
Unless the problem is still small, I usually get rid of all the lettuce and sanitize the area as much as possible. Since aphids have multiple hosts, if I spray, I spray everything nearby. I plant lettuce in a different location next time and I usually wait a month before I will do that to make sure the aphids are gone. Sick lettuce rarely gets better and it is a short crop so I usually just start over.
Happy gardening in Hawaii. Gardens are where people grow.