At first glance it appears to be slime mold, which is ugly but relatively harmless. Looking closer there is something going on in your grass that appears to be a more harmful disease process.

Where do you live?

What kind of grass do you have?

What is your watering schedule? How often and for how long do you water? Have you gotten much rain recently?

I like in Clive, Iowa.

I have 50/50 mix of Kentucky Bluegrass and Midwest Rye grass.

Yes we were getting rain ~4inch this month (source: https://www.wunderground.com/history/airport/KDSM/2018/6/6/CustomHistory.html?dayend=23&monthend=6&yearend=2018&req_city=&req_state=&req_statename=&reqdb.zip=&reqdb.magic=&reqdb.wmo=). I water one inch every 2 days if no rain.

Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the daytime air temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall.

"My hose, sprinkler and water pressure takes 8 full hours to fill the cans."

Is that mean if temp is 90s then you water for 8 hrs once a week?

In other words deep watering given 1 inch on single session?

That is what that means. It also means in the spring I water once a month for awhile, then once every 3 week, then every other week. I do that unless it rains, which we got lucky this spring and got 2-4 inches of rain every month until May. So I did not water at all until a few weeks ago.

What I did not cover in that little paragraph on watering is how to transition from frequent watering to deep watering. When you water frequently your grass will have short roots which dry out faster than infrequently watered (deep) roots. You should ease into a deep and infrequent pattern. Start by going deep and backing off on the frequency as your grass tells you. It will show signs of wilting in the afternoon. Quite often the same grass will look fine in the morning but wilt again the next afternoon. If that pattern continues for 3 days, I'd water it. Usually it will get to looking wilted in the morning. That is the most definitely time to water. Once the roots become more established in the deeper soil, then the wilting effect should go away.

Thanks. That helps lot to understand.

BTW back to my original question. So in that picture is it a fungus or mold on my grass? If so do I have to put some thing to cure it or it just a watering issue?

Thanks. I will try it.