it will decrease, but it will not go away unless you do those things I said...
Macro algaes for reef tanks are the equvelant of our seaweeds and the most commonly available one in the NZ hobby is Caulerpa. Here is a good article about them: http://reefkeeping.com/issues/2007-01/sl/index.php.
I think Astrea snails will eat cyano, you will probably need to find a hobbiest that has some they don't want anymore.
The truth is that the Marine hobby is expensive and to run a successful tank (that is Healthy and free of excessive algae) you need to invest a decent sum of money. If you do not have the money requred for a decent protein skimmer I would suggest researching the 'ecosystem method'. I have a small 250L reef tank which has cost me over $3000, and most of my equipment is second hand. Admittedly alot of that is spent on expensive lifestock, but good equipment is vital for healthy lifestock.
With regards to tap water, it is not the chlorine etc we are worried about in Marine systems it is everything else, such as Silicates, which don't matter much in Freshwater tanks but can cause algae growth in Marine systems.
I suggest you read the sticky here if you haven't already.
Cheers- David