Aerobic, anaerobic and facultative bacteria have gone to work work in your little pond and soon the water will smell like rotten eggs. My advice is to empty the water out and use it as a planter. This is too small for a water feature.

it is certainly not happy but I'd be tempted to lay newspaper or paper towels on the surface and the 'scum' will stick to it, then carefully peel it back and then off. This may be enough. or consider a small aeration unit. There are many small barrel water features for sale like this but they have a system for gently bubbling water to reduce the risk of it going smelly.

The only other option is to dump out the old water and refill on a weekly basis. Bubbliizers require an air pump, tubing and electrical connection which may not be safe for outdoor use.

Here in the desert, we usually drop a small immersible fountain pump into the tub on one side, with the outlet pointing up. That recirculates the water and helps keep the lower levels well oxygenated. As Loosestrife says, though, that requires installing a weatherproof electrical outlet near the pond.

You can get solar powered ones that may be perfect, after all you dont want too much movement if its for wildlife.

Thank you all for your suggestions. I will try getting scum off surface then invest in a solar aeration unit.

I had a Half Barrel water feature that did very well until the barrel collapsed, best to line it first, I had a Water Lily & Oxegenating Plants & it looked after it's self, I added rain water, not tap water.
Birds used to plonk themselves on the Lily pads & have a lovely bath.
So I would put plants in like I put in mine.
I have started a new one & at the moment only have a Oxegenating plant in it & its clear.
Best of Luck x

Looks like the barrel has a preformed liner insert in it.

Am pleased to report that the water has cleared up a bit and I have added more oxygenating plants. Yes the barrel has a liner in it. Someone at work said it needs to go ‘through a cycle’, then it would settle.

Years ago after seeing an article in a magazine about a Half Barrel pond .Idid the same .But it was sinking the Barrel more into the ground Then I filled it with water , Oxygenated plants ,a few Reed type plants in pond .2 days after we were amazed how many Frogs found out that there was “a New pond to explore” .kept it for quite few years ,then redid our garden & put a proper bigger moulded pond in .