I am sorry to learn of your loss. The Social Security Administration should have been informed of your wife's passing, and her checks should have stopped. If she received an excess check in April, then that should be repaid. Check with the Social Security office to be sure that all is OK with her account.
Because you say that your are both "green card holders", your status would that of a RESIDENT Alien for U.S tax purposes. Because your wife passed in 2018, then you would file with status of Married Filing Jointly (MFJ)--if you need to file. You would include all of your 2018 income and as well as your wife's income for the year. The gross income threshold for filing for a 2018 MFJ tax return (both over age 65), is $25,300 based on income which is not exempt from tax. So with the income that you indicate (and assuming that you stated all of your combined income), then you are well below the filing requirement. It does not appear that you have a requirement to file. However, you can file if you want to file. Because I don't know how well the IRS connects with Social Security, some of my friends tell me that filing the return, and marking your spouse as deceased, might be a good practice to help stop future fraud attempts. You would not owe any tax money for 2018 with the income that you stated.