Hey Camaro did you have a long night last night. Every time i write something YOU have something to say and in fact purple sponge algae is exactly that a PURPLE SPONGE -- i thought you would know being an "experienced" aquarist that some sponges appear green because algae clings to them. The algae provide oxygen for the sponge and the sponge provides carbon dioxide for the algae. When two living things live off of one another, it is called symbiosis. which is that same for purple, pink, green, brown, or any color for that matter. so by me saying " purple sponge algae" would mean PURPLE ALGAE stick to a SPONGE. Now you do know what a sponge is right? and by the way what did you mean when you said "I think you better hang on to the woman... she looks nice to have around" (in a different post)- it really doesnt make sense if it was directed toward's me. Back to the topic - it is basically Purple Plate Coralline Algae, or ( so there are no miscommuntication )Mesophyllum sp. stuck to sponge. I hope i answered your ? well enough for you (By the way dont question my intelligence ) Hope you sleep good tonight buddy. -- Maybe you wont be so argumenative tomorrow.