It is normal, in the summer it can be impossible to prevent it going green as it happens so quickly, I know many that will leave fresh clean water in their stables preferring to drink the green water in the field troughs, it is not easy to scrub out automatic or very large troughs as they need draining first, just do what you can to prevent it getting too bad and make sure they are drinking regularly.
Yes I have a large metal field trough. It weighs a ton, so impossible to pick it up with water in it. I guess when the ground is dryer I could let it go low and tip it up and then scrub it, but there is no way I can do that now, the ground around it is too wet. They don't live in stables so I cant put it in there. So im thinking it will be ok, it just looks yuk.
We have large sheep/cattle troughs with automatic feeders. I don't mind a bit of a green tinge, but if there's larger algae growing or too much crud settling at the bottom, I will clean it. My method involves tying up the floater arm (so water doesn't refill) and emptying the trough out with buckets until there's a few inches left. Scrub well with old dandy brush, then scoop out the rest of the water with bucket, then rectangular dustpan (very good shape for scraping along flat bottom and getting into corners). Use sponge in the end to get the last bit of gunk out. Then untie arm, let refill. Takes 15min tops.
Of course, you end up with lots of water around the trough, but frankly, it's been so wet until recently that a bit more probably wouldn't have mad a difference.