snip
Can anyone advise if this is actually an algae infestation, how serious/dangerous this is and what sort of action i might take to fix it?
snip

Looks like algae. You cannot eliminate algae, they are ubiquitous. You can control it by making anything that comes into contact with nutrient opaque.

It won't harm your plants. It does consume nutrients, though.

Our NFT system has algae too...and now that it is systemic, we can't get rid of it. However, I have found that flushing out your system on a regular basis is very helpful.
We have a re-circulating system with a gate valve that allows us to drain out the system if necessary. I shut off our nutrient and acid dosing pumps and just let it purge itself all day. While it drains, I flush out our distribution manifolds and spray out the gutters. Although you go through more nutrients annually, I think that it is very helpful in clearing out the accumulation of algae.
Also, I suspect that the algae not only uses nutrients but may even use them differently than the product you are trying to grow would. I would think that it can leave a disproportion in nutrients from your original nutritional formula...and if you read the EC, it can say the "right" numbers but lacks what you really need in the system. Because of that, it is all the better to clean it often.

drain the system and clean it, and light proof your res.