Newbie questions

Hey everyone,

I'm a rookie at all this so trying to learn the ropes and make as few mistakes as possible.  I recently purchased a very well established 125 gallon tank with plenty of live rock and corals.  I don't have fish but have some shrimp so far.  Whenever I run water quality tests they come out stellar.  (salinity, ph, nitrate, nitrite, ammonia, and calcium).  I have had the tank for about 3 weeks now and roughly a week and a half ago a reddish brownish material started to surface on the substrate which is very very fine sand, but that's the only place.  It does not appear slimy or form as a mat, but almost seems like a very fine grainular texture at least by the looks.  I have only seen red slime algea in a tank that was badly overrun.  Is this what it looks like in the begining? Or is this simply from not having anything turning up the substrate yet?  It seems to start  in the areas that have lower water flow.  So far I have vacumed the bulk of it off twice while performing water changes which I have been changing 10-15% weekly.  Being I have no fish, the food has been minimal so I have not been overfeeding and I have a very good protein skimmer.  Any suggestions?  My other questions is tonight I noticed on one of my larger pieces of live rock which is completly covered in coraline algae that it had small white/or at least paler spots all over it.  I had not noticed it before and was wondering if this could be a potential sign of something I should be treating.  It seems to be on that rock alone and not on any of the neighboring rocks.  Again, it may have always been that way I just hadn't noticed, but I've been eye balling the tank the last three weeks ; ) and just noticed it tonight.  Any help would be very appreciated.
Paul