I've had this happen and heard that it can happen - for a variety of reasons - it can happen to all sorts of algae.
The problem is that as it dies it releases everything back into the water and can almost trigger a cycle-like blip in nitrites etc.. so you might want to pull it out of there. This is what I have ended up doing in the past - I take it out, pull it apart to break out some healthy looking pieces, then throw them back into the sump: they always seem to grow back into a healthy mass.