Welcome to Terra Forums! I hope I can help you with your problem.
It looks like your plant is not in a free draining pot, is that right? I am going to assume that your water is distilled, rain, or reverse osmosis, and that you understand the need for a salt and mineral free substrate?
The fact that the plant was previously growing well in your conditions leads me to conclude that you are expeiencing a salt concentration in your mix which has increased over time.
The fact that you mention your use of perlite and that it is turning green leads me to think that the growth of algae and mosses is being supported by micronutrients present in the peat and perlite that you have used, and that these salts are being concentrated over time as evaporation and capillary action brings them to the surface of the mix.
I always advise that growers rinse their potting materials. This is based on several growers analysis of the runoff water from unrinsed peat. The concentration of total dissolved solids was recorded using a standard TDS meter by 3 independent studies, and showed the TDS levels to be between 300-1000 PPM. It is recommended that such levels not be above 100 PPM!
For this reason, all my Drosera are in pre-rinsed materials. I find the occurence of algae and moss is frequently associated with the use of perlite, so for this reason I prefer silica sand in my culture of these plants. I also grow them in pots, never in a substrate terrarium because I have found salt build up in this situation is inevitable. Various blue green algae actually fix nitrogen into the mix, so over time the PH rises in a process that releases stored nutrients from the peat, and the plants do not appreciate it.
Once the proliferation of anaerobic bacteria starts, sensitive plant tissue like the root tips are attacked. As long as a droserae have a healthy growing root, then there will be a happy growing plant. If the lower levels of your terrarium or undrained pot become void of oxygen, then root tip problems are very possible. This is why I allow my plants to sit for a day or two in dry trays: it allows air to reach the deeper layers of the pot where the root tips are.
For all its size and commonality, Drosera capensis seems quite sensitive to adverse salt accumulation. I would suggest that if it is not in a pot, that you transplant into clean fresh mix in a pot. If you begin to notice the green glop, this may then be leached out by dedicated syringing at least daily. The condition will resolve if you stick to the syringing, allowing the water to drain out and away of the bottom of the pot.
OF course, there may be other factors involved, but this is what comes to mind. I have to stress that I don't trust perlite. Sometimes I have no problems with it, other times.....ick! I don't know if individual batches vary in composition, but I do know that I no longer trust it, and if I do use it I rinse the heck out of it.
The issue of pre-rinsing potting materials has been debated. For me, the proof of it is in seeing the difference in health that comes from using this protocol.
Anyway, here's my question: A lot of the older leaves stopped producing dew and have started to droop a little (which I've read is normal), but the new sprouts from the center have been a light brown color at their bending point, and then turning black at the VERY tip of the leaf. I assume this isn't normal, but I'm not sure if the cause is too much water, too little, or something else. The new growth that has already opened up have been very slow to open and have not had full development of the red "fingers" that the dew is on. One leaf curled backwards into itself immediately after opening completely. There aren't any bugs in the terrarium, and the temps are from 72-82 depending on time of day, and the relative humidity is ~80% constantly. There are 3 20 watt grow bulbs about 6 inches from the top of the plant, and since there has been so much growth, I assume light is not an issue.
