Replaced pc bulbs, red slime out break

first off iam new here... i have have my 48gal corner tank set up for about 4-5 years now, just live rock and a few dam,and a bandit shrimp, anyway the other day i messed up bad and did a 20gal water change, and put all fresh water back in, salt droped low it took about 25 gal of salt mix to get it back up, over a weeks time at two gal at a time. after that, i noticed red turf,on some rocks,so i started thinking it has been 4-5 year sence i changed my pc bulbs, so i did, three days later the red turf turned into red slime,iam at a loss ... my tank is at 79 temp nitrate are at 10,ph is 8.2, salt is at 0.25, also i have a 800gph pump hooked to a squid wave maker that has a cross current setup.was it the light, or other factors...thanks Bill

great thanks for the reply

red turning green

its been almost two weeks when i put the new bulbs in ,and the red slime is turning green, is this normal, salt 0.23 ph 8.2 water temp 79 nitrate 20 doing a 5gal water change today, its a 48 gal corner tank with 130 watts pc litghting set up far at least 4.5 years,,, pc bulbs had never been changed scence tank was set up, with what happend with the fresh water thing and the new lights, is it possibal thats its trying to re cycle....