Lettuce should be a low ball of green - even young. You have insufficient light. Can you move your hydroponic operation outside? Lettuce is a cool weather crop, so southern California might be warm enough now to have lettuce outdoors.
The plum tree would do better directly planted. Also, if your soil is alkaline (many soils west of the Mississippi are alkaline), then your plum tree might enjoy a bit of sulpher or something to lower the pH.
Perfect. I just set up my 5X8 greenhouse in the backyard today and moved the lettuce out there, along with my basil. In a little while I’m going to add the red potatoes and a small raspberry bush that looks kinda like a bareroot rose coming out of dormancy.
Soon to be planted are jalapenos, onions, cilantro, roma tomatoes, cucumbers, broccoli and carrots. With all that variety something has to grow, right?