What is it and why is it there? It could be a variety of things, and I couldn't tell you without a picture or a decent description. It most likely is either Cyanobacteria, Dinoflagellates, or Diatoms. All of which are unicellular algae.
What purpose does it serve to the tank? It helps the tank by taking up nutrients that the bacteria can't keep up with yet. In more established tanks, they are a tale tell sign that something either went wrong recently or has been slightly wrong for a while. Excess nutrients is usually that problem.
Is there something I can do to hurry it along. Honestly? No. You can quickly get rid of it now with water changes and other nutrient export, or if it is Cyanobacteria you can use products like ChemiClean to kill it. But any of those above methods gets rid of an aquariums way of getting rid of excess nutrients. If you get rid of it before all the nutrients are taken up, it will only come back later.
Will it affect my fish? That's a loaded question. Cyanobacteria, Dinoflagellates, and Diatoms are a broad family of unicellular algae. There are good and bad. In fact, a specific dinoflagellate is what helps Blue Ring Octopuses produce their Tetrodotoxin to make them so deadly, but I doubt that is what you have. In any case, it is common place to have these above algaes in your tank and 99.9% of the time, they are just harmless cycles of an establishing tank.