So, if it is grackle poop, it's not really going to hurt your lawn. Technically speaking, grackle poop could be a bit "hot" for the grass - in the same way that chicken manure is too hot before it is composted to put on plants - but spraying it into the lawn (I'm assuming you mean spraying water on it so that it washes down into the lawn) will likely work out just fine. Manure has a lot of nitrogen in it and, of course, plants taken in nitrogen and use it.
I would encourage you to take a tip from my neighbor who utilizes "flashy" things to discourage the birds from messing up his garden. He uses things like aluminum pie pans, that yellow "caution" tape and streamers. The birds (including grackles and crows) don't tend to like those things blowing in the wind and moving around. It really does work quite well.
Now, yellow caution tape and aluminum pans might not be something you want in your yard but you can put something that is more attractive up that has the same effect. I've seen folks put up some things like wind chimes or yard ornaments that look pretty nice. This might do the trick. The birds may just decide to go elsewhere if they are annoyed/concerned by those flashy things.
I have a farm and at one point the barn was home to a number of birds. They were crapping on my tractors and other things in the barn. I tried one of those fake owls - didn't work, there was bird crap all over the top of it. I also bought one of those multi-hundred dollar systems that purport to scare birds off using a high-frequency sound - that didn't work either. In the end the only thing that worked in the barn were some barn cats. This likely isn't a solution for you though - and you'd be exchanging bird mess for cat mess - but my point is that the birds left because, in this case, the cats made the barn a place that they didn't like. Perhaps putting some things up in the yard will cause them to look elsewhere too.
Best of luck!