I am having a hard time interpreting your pics. Yes, I see string algae. If there is a pic of old dried yet fluffy stuff looking more like weeds, then grab it with a rake and its out of there. If its still underwater and slippery as one would expect, perhaps use a toilet brush taped to a pole. The pics don't make me think the pond is as big as you indicate. What are the outer dimensions? Where are you, what zone?

If you search other threads here, you will find several that suggest that some string algae is a good thing to prevent the pea soup type. Speaking of the pea soup algae, I hear that it is the only type that will be killed with a UV light tube since solid stuff wouldn't flow thru the light tube.

You could sure do with more plants. I use them for four things: beauty and edge softening; nutrient usage; pond shading; and I have a 30" stream with plants that is essentially a monster filter of roots. The extra shading and nutrient uptake would likely slow your algae.

I have had a koi pond for over 16 years and suffered with this string algae every spring. My current pond is 4000 gallons. Last year I purchased a couple of hi fin sharks per someone's recommendation. No string algae last year and no string algae this year. It's incredible how they take care of it. I have two but plan on putting in one more.