no no no, do not do a gravel siphion, I don't do a gravel siphion no more then once a month, just changing the water will be fine, if you do to many siphons then your breaking down. I remember when my pleco was doing that, and it was becuase my heater cracked and stoped working, whats your temperature in the tank? Also the pleco may not be geting enough nutriants, try throwing some cucumber in the tank, and if you can find a peice of driftwood I would throw that in thier as well, plecos will eat that stuff like icecream. If none of those things work, then take a water sample and post it here. they only get moldy like that then they are really stressed out, its just some kind of reaction to stress. If all of your water prams are ok then I would go ahead and asume it has a sickness, It might me a fungus so I would treat it with some aquarium salt, a little higher temperatures and add just a little melafix and see if hye gets any better.