Sounds like a fungus to me. Quarantine it as best you can, buy a fungicide (a Sulfur-Based fungicide is safe for CPs, so, try that) and, if you're watering via the tray method, make sure the tray is empty for a day or so before filling it back up again.
If that doesnt work, toss it, D. Capensis makes seeds like it is its job, so, you'll get more than you know what to do with soon enough.

Ugh!

While quarantining the Sundew from the terrarium I got a look at its roots they were nearly pitch-black! It died from root rot. I'm not sure why though. I keep the media moisture of the all the Sundews at about moderately to very moist and the other two grow perfectly fine in the conditions I give them.