Hello Subsea,
yes, the Brown algae is a Sargassum, which really good grows in this tank but colors the water a bit, i think.
The "bright red thing" ist a sponge.

I don't really say corals and algae are not compatible, but a lot of them grow like mad and have the tendency to overgrow corals.
You always have to cut and control, just like in a FW tank.
Some red algaes are better for this, but i guess there's always a kind of chemical war going on between corals and macros and these often win the battle.
But this may be because i like running my tanks skimmerless, only with GAC.
A skimmer would maybe be better in this case, but i'm from the "Plenum old School", can't help it.
Back to seagrasses, I remember a couple of people, who did a very good job with it, once upon a time.
Especially Sarah Lardizabal, who wrote this awesome article and also spoke about her tank here on RC:
http://www.reefkeeping.com/issues/2006-04/sl/