I'm not exactly sure what it is, I had a frond that did that as well earlier in the season. I kind of stood back and monitored it - It only happened the one time and every frond since then has been fine. Maybe someone on here can give a better discription of what it is exactly.

I had this on 2 fronds on one of my windmills, and figured it was probably that way before I purchased it. I noticed it weeks before they began to spread, and it was a decent bit worse than yours. Don't know what caused it, but since then the two affected fronds have grown and spread completely as if nothing was wrong, so it seems it's not too big a deal.

Thanks for the replies! I'll continue to monitor it.