Your temps sound good it must be over watering. The rockwool should be moist but not soaking wet. A little bit of green algea is not bad but slime or any smell is bad. Do they not have a way to drain your table ? you might have to get a tube or siphon like for fish tanks and sneak the tube in to siphon off the water. You have to change the water every 7 to 10 days to keep fresh water and nutrients in there . How long has the water been in there ? We are getting close. I know they say 24/7 but you may have to get a timer and go 30 minutes on and 30 off or NFT's go 15 on 45 off when lights are not on. The long water roots can hang in the water 24/7 but the root ball that's in the rockwool needs to be mostly dry. rootballs like oxygen more than water. First you must be able to change water. toxic salts build up from what the plants give off in the water and poisons the water itself. you got a nice space and should not have any prob's. Your light can be closer. If you can hold your hand at the top of your plants for a minute and not burn it's alright so get the light down as much as you can. OH and if you don't have cal-mag use 1 to 2 teaspoons of epson salts per gallon of water till you get the cal-mag. Also 1 to2 teaspoons of h2o2 (hydrogen peroxide) per gallon will help to keep it clean too. I really think it's the watering that's got you but we will get you through it. Hang in there m8. Anyone familar with this system come on down. A multi timer can be set for 15 minutes at a time for 24 hours. I have one that I set for 15 on 45 off for my vents because of the cold right now. It doesn't run during dark they don't need fresh cold air when dark. Again hang in there m8 they don't look that bad yet, they can recover.