The dark, red algae is probably not algae at all, but is cyanobacteria(slime) and should dissipate through time. Try to syphon it out of your tank and watch excessive feeding and increase wate rflow for optimal results.
Now for the Sea Apple...I would not keep a Sea Apple as it will release toxic gasses when stressed or dies!
Aquarium Invertebrates
"Death in a colorful package"
I just wouldn't chance it personally!
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125gal.w/Mag9.5 return(dual megaflow)>Mag7 pump Aqua Cev180skimmer.Wave2k Hamilton Reefstar(2)250watthqi(mh)pend.a Yellow, Naso Tang Red Lip Blenny Percula Clown Demoiselles Niger Trigger F. Wrasses Ceriantharia Orn.Shrimp and Stars Hermits Queen Conch asst. snails> Stars Zoos shrooms Montipora Brains Gorgonians Favia Turbinaria(large+small polyp) Acropora Xenia Tridacna (CroceaMaximaSquamosa) Leathers <35+75gal.reef tank as well>
"IF THE PHONE DOESN'T RING...IT'S ME"

jb