Live rock turning white

I new to the hobby of keeping a tank. I have only been at it for about 4 months and two of that was waiting for the tank to cycle. Lately my live rock has been turning white. The white parts are very hard and I cannot scrape it off. It kind of looks like hard toothpaste.

Water changes performed twice a month.

My water params as of 3/13/10. This is all I test for:

temp: 79
SG: 1.022
ammonia: 0
pH: 8.4
dKH: 12 I'm not sure how to read this test yet.
Calcium: 380
Nitrite: 0
Nitrate: 20
Phosphate: 0.5

lighting:
T5 HO 54watt
Coralife Actinic Trueblue 28watt

Skimmer: AquaC Ramora hang on

 

if you had the rock out exposed to the air for a lengthy time the coraline that was on it would die off. dont worry it will come back.... i had none now i hate it... my tank is purple :/

 

It's coralline algae that is more than likely dieing off. Have you tried to raise your Calcium up around 420? 380 is a little low imo. If you're unsure how to go about it let us know and we'll help all we can. What type of test are you using for Alk? (dKH) maybe I or we can explain how to read it to you.

 

i had purchased the tank and everything from a friend. after doing some research I found out the test was an API test kit. The last four number stamped on the bottle are 1008, which come to find is basically the date it was manufactured. so i think its still good for one more year.

im still new to this hobby and had to research how to do the test. i put one drop in then it changes color then i count the drops ect. ect. Well it takes about 11 drops to change to yellow, but i didnt know if that was meq/l or Dkh I was reading? its the Dkh right?