I have that on one of my plants, too...I got the plants from my friend's tank. Once it got into my tank it started going away (luckily it didn't spread!). I think my friend doesn't do frequent water changes (monthly I think) so if an unstable CO2 is the case...maybe all his fish waste is causing excess. I'm going to google it as well.
The one good article I found said that it's hard to get rid of and suggested picking off the leaves or scraping it off with a credit card (and watch for it on gravel). I am going to go out on a limb and suggest a couple of things:
1. "siesta" put your lights down for nap during the day for a few hours, mid-day. The rationale for this on a previous thread was that algae photosynthesis takes 4+ hours to start so if you do lights out after 4 hours, then put them back on a couple hours later, the algae photosynthesis process has to start over.
2. If you only have a couple of infected plants (blackbeard tends to like the wider leaf plants, and it's only on my Java Fern
so far), you can try bleaching them. Here is how I bleached my brown algae without compromising the health of my plants (or fish):
I prepared three bowls: one with a 1:20 concentration of bleach:water. Bowl #2 had clean treated tap water. Bowl #3 had clean treated tap water.
I pulled out my infected plants and put them, leaves only (not roots) into the bleach water. Left it dunked in there for about 3-5 minutes. Took it out, rinsed it thoroughly in bowl #2, then dried with paper towels. Rinsed thoroughly in bowl #3, then paper towel dried again. I would even say use 4 bowls if you are doing lots of plants.
It's a risk, but I think if you rinse off the bleach thoroughly with treated water, it should be okay.