Can anyone tell me what this is? Is it algae or a chemical build up or something?? Do i need to be worried? It has a blue tint to it...

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looks like some form of algae.

Specifically, blue-green algae (BGA). Actually not an algae at all, it's a photosynthetic bacteria.

I find it necessary to stir up the substrate near the glass on a regular basis. You can use a chopstick or an old credit card. You should clean your substrate now to get rid of this build up.

will it do harm down there?

if allowed to remain it will migrate other places so it's best to deal with it early (imo)

I was just abit worried that it could be something toxic , as it's blue , but if it won't do any harm down there then i could just leave it .....

I just push an old credit card between the tank wall and the substrate and it both buries it and scrapes it down away from the surface. Been doing it for years now and I haven't had BGA become a plague again for a decade now.

If it is in the substrate but not a plague in the rest of the tank you doing just fine. It may appear in other less tended areas of the tank. I have had it in the slots and back of my overflow box for instance but it hasn't invaded the floaters, the surface of the substrate, on the plants or wood. Clean the overflow box and it may not appear again for months.

Most remove it to remove the risk
Like I said remove it but don't be overly concerned seeing it.
If you read any details on what BGA actually is I think you will remove it rather than allow it to remain in the tank.

Ok I'll give it a clean , cheers for the info guys