my prints have a hazy psychedelic mottled surface. Someone told me it is because I have them in the developer for only a few seconds and they need to be in for 30 seconds, but then they would get too dark. Should I dilute the developer more or close down my aperture or decrease my exposure?
It takes time for development to come to completion. If you pull the print out of the developer too quickly, you will get a blemished that is dappled, we call it blotchy. As a rule of thumb, print development time averages about 90 seconds. Some developers are more energetic yielding a developing time of 60 seconds. Others are sluggish with a developing time of 120 seconds. Time in solution is a variable based on temperature and chemical strength. I often told my students to plop the print into the developer upside-down and wait 90 seconds before turning them over for viewing. This way, they quickly got it, you must adjust the enlarger exposure time so that the prints develop up within the recommended time span = 60 to 120 seconds.
Without seeing the print, it sounds like uneven development resulting from a very short development time. What's your developer and paper, and what are the recommended dilutions and times? Most print developers have a one to two minute development time at the recommended dilution.
If your source image is underexposed, you would control this not by decreasing your print development time but rather by using shorter print times and/or stopping down (at the enlarger). It could be that it's so underexposed that a well exposed print is unattainable in the darkroom.
If you need to pull your print from developer before it gets too dark then you are overexposing it. It will look no good; prints are supposed to be developed till completion.
Dilution of the developer is not the problem, provided you did stick to the manufacturer recommended dilution.
You need to close the aperture and/or shorten your time. Or some combination of both - remember that 1 step of aperture means doubling (halving) your exposure time.