if it is slimy it could be cyano algae which is really a bacteria and not a true algae, usually it is dark red and some call it red slime alage. This is usually the result of overfeeding (high nutrients) and not enough flow.......the start to tackle this should be as Joe suggested, get some of the water tested and give us the exact parameters, not just that an lfs said they were fine......Nitrates, phosphates, ph, temp, salinity, akh, calcium. What kind of equipment is on the tank, size, powerheads, lighting etc. You should try to remove as much as you can by siphoning out and doing a major water change......matching the existing parameters as far as ph, temp, salinity and akh as possible, mix the water 24 hours ahead of time and do about 25%. then reduce the lighting period, increase the flow if possible and don't feed anything for a couple of days, then start feeding again, every other day......they wont' starve, they will scrounge for food amongst the rocks etc.......good luck and let us know what happens...