It's that time of year, Springtime. It's when your pond is transitioning from winter. You're going to get some algae until it all balances out.
I filter with a bog and the water is crystal clear almost all year. This is the only time (Springtime) when I get cloudy water and algae. Once my plants wake up, it all clears up. It usually takes about a month. Plants are a big part of the ecosystem.
Your fish have woken up, are eating and producing waste creating an excess of nutrients. Try not to overfeed them.
Nature protects the ecosystem by adding algae (plants). Get some plants in there. Lots of them. They will starve out the algae by competing against them.
You can throw floating plants in there. They are easy. Just toss them in. Plus they can shade the sun which may help against the algae.
You can stick plants between the rocks with their bare roots in contact with the water.
If you add plants that are in pots, replant them in mesh planting bags. The roots will have better contact with the water.
You can also get floating rings that can hold plants.
Of course, your filter does matter too, but if it was working well last summer, I'm guessing things will clear up in a month or so.
I know it's hard, but try to keep the fish population down. The more fish the more waste. Plus, as they grow larger, they produce more waste.
It's a balancing act. Filtration, plants, fish and sunlight to a degree.