Help! Opae ula and brown algae?

[snicker07]

Hi, I transferred 15 opae ula into a 3gallon tank using disstilled water, artisan water and aquarium salt. I have kept the specific gravity at 1.010. This was done in September.
They have been doing great. I have a small amount of hawaiian sand and live sand from petstore mixed and placed just enough to barely cover bottom of tank. I placed a large piece of coral that i picked up on a beach in mexico 5 years ago and some branchybrown coral and black lava rock that was in a ecosystem with them. The aquarium is in a south bedroom but gets very little if any direct sunlight. There is a led light that I turn on and off that the aquarium came with. Also a small heater to keep water around 74degrees.
Last week I noticed some small light brown circles showing up on the heater side of the tank....now this week is is spreading rapidly..there are lots spots in all sizes and it appears that the top side of the coral rock is becoming discolored.
I usually feed pure spirulina powder once a week and I skipped last week when I noticed the brown stuff..
What should I do??? A partial water change? Clean inside of tank with sponge tool? add an algecide that is safe for inverbrates? Any advice would be welcome! I enjoy the little guys so much I just can't figure out why I would have this trouble 2 1/2 months into this hobby.
Thank you!

[guavakai]

hi
that's how my algae started forming. i just leave it, and now only scrub down on the front sides so i can see the shrimps.

[snicker07]

Wow I wanted to post some pictures but when I tried the came out terrible.