Vanessa, I think you need to gently slide the plant out of the pot and have a good look at the root ball. The green on the soil surface might be algae which is quite normal on a plant that likes to be in the shade. Don't worry about that. The flowers look like they have wilted, not dried up as if they are finished blooming. I think the plant (soil) is too dry.
But when you pull the plant out, (lay it sideways on a sheet of newspaper or something to keep the soil from going everywhere) if the soil stays in a solid block, and is mostly dry, you need to loosen it up a bit by sort of crushing the block of soil on all sides (gently so as not to break roots) then replace it back in the pot with any loosened soil, and soak it thoroughly for an hour or two, to re-moisten all the soil. Put it in the sink, or in a large bowl or bucket. Room temperature water is best.
Then put it in a saucer or on an old pie plate so you can see it drain. (well you might need to let it drain in the sink for a few minutes or the saucer could overflow.) The water should never go straight through and come out the bottom right away. It should take a few minutes and a fair amount of water to wet all the soil completely, and have some then drain out.
If you are living somewhere that has hot weather right now, and your a/c is running the air is pretty dry too. Hydrangeas really like some humidity so if you have a shady place outside, it might be happier there, too. (edited to add) Just checked your pictures again, and if that plant is getting direct sunlight through that window, you need to move it away so that it's out of the direct sun. Hydrangeas like shade. Near the window but out of the direct line of the sun would be fine.
