carbon shouldn't "release" stuff back into the tank, but it will saturate and stop being helpful. This is of course dependent on many factors...ie quality of carbon, amount used, nutrient levels in the tank, flow through the carbon, ect...If changed out regularly, there shouldn't be a problem. I would hazard a guess however that if the carbon was totally saturated with stuff, a small amount may be re-released into the tank depending on flow, or in a high flow situation a particle of carbon may eventually degrade a bit causing the same. Also, the carbon is going to act just like any surface in the tank, you'll have some biological activity going on there, primalrily on the surface of course...so some of the nutrients absorbed will be converted into other things like Nitrate and such...another good reason to change it out regularly.
What about a phosphate remover?
Yes, another good tool in fighting algae/cyano outbreaks. There are two main types, ferrous (iron) based and aluminium based phosphate removers. There is some debate on both...In a nutshell, some avoid aluminium based ones because of fears that aluminium will leech into the tank, which has been shown to be a photoinhibitor in corals. There is debate, however on the degree to which this happens and even if the form of aluminum in the remover is harmful in the first place. Ferrous based ones don't have this issue, but they tend to be much more expensive, and usually require use in a reactor of some sort. Also, iron leeched from them could become algae fuel.
Having said all that...I've used aluminium based phoshate removers pretty much 24/7 in my reef for many years, with no observed ill effects on my corals. HOWEVER...please take the time to research them both yourself before making a decision on which one to use...
MikeS