Barnacle problems?

A number of years ago I had a 100 gallon reef tank. I had a pretty typical
setup, wet-dry filter, large home made skimmer, some kind of auxillary
power-filter & secondary skimmer combo thing hanging on the back, about 5 or
6 floresent tubes. This was my first venture into reef aquaria. My local
dealer who was fairly knowledgeable had ordered some live rock for me from
an unknown (to me) source when I first established the tank. The tank did
fine for about 6 months, I was successfully keeping mostly fish with a
couple corals and an anemone. Then I started noticing that these little
barnacle-like creatures in the tank were increasing in size and number. They
had a brown or grey hard shell and suction-cup body underneath, and they
would stick themselves to the rocks and sides of the tank and would be quite
tough to remove, I usually had to scrape them off or pry them by hand. After
a few months they started to get out of control, I would spend hours every
couple of days removing probably hundreds of them, and no matter how many I
removed there would be plenty more in a couple of days. They would grow and
then die off by the dozens, so naturally they were causing problems by
decaying inside the rocks and impossible-to-reach places in the tank. After
about a year I could no longer keep up with it and it led to unhealthy water
conditions, a constant unmanagable thick green slime-like algae, and the
eventual demise of the reef. I moved most of my surviving fish and inverts
to my other tanks. It worked out okay in the end because I was moving across
the country and had to sell my larger tanks.

Anyone else ever have any problems with these barnacle-like things? Any idea
what they are? I am considering going (very carefully) back into reef
aquariums (although this time armed with a little more experience,
knowledge, and a high-tech setup) once I get my new house built and will
obviously need to have live rock again, but am a little afraid of bringing
back these nasty little beasts and going through it all over again.

Thanks,
Chris