I have for the first time opened QGIS 3 on my Mac. I am trying to create a rectangle grid of 50mx50m cells. I then want the centroid of each cell of the grid shown in the attribute table. I can get to this using a scratch layer but the format of the attribute table is shown in metres not decimal degrees. 
QGIS Cannot show Grid Layer Attribute Table variables as Degrees
attribute-tableqgis
Related Solutions
This is a kludge but it does work - haven't found a way to go directly from points to raster yet (but am hoping someone gives a solution here!).
Starting with a point grid (random points in the Serengeti from the Vector|Research tools|Random points tool):
Create a polygonal grid of the same extent and cell size as the raster you'd like to have (this from Vector|Research tools|Vector grid):
Use Vector|Analysis tools|Points in polygon, using the polygon grid and the point layer:
This gives you a new grid (here colour coded by the number of points in each polygon cell):
Now use Raster|Conversion|Rasterize to convert this to a raster, using the PNTCNT attribute for the raster values and the same cell size as chosend for the grid:
... giving a raster with values as per the points in each cell:
You have to set your Project CRS to something using meters, like UTM or State plane. Then you can use Vector -> Research Tools -> Vector Grid ... to create a grid in the same projection.
You should set the X parameter to something useful, e.g. 150 for a 150m-grid.
Best Answer
Firstly, it looks like you created your grid in EPSG:3857 Pseudo Mercator. Although it uses meters, it is not recommended for analysis or accurate distance measurements. I would suggest a local UTM coordinate reference system such as WGS84 / UTM zone 18N EPSG:32618. You can select this in the algorithm dialog when you create your grid. Click the CRS selector button (shown in the screenshot below) to open CRS selection Dialog.
You can filter the list by typing (e.g. 'utm zone 18') into the search bar at the top of the dialog (see below). Select the CRS and click OK.
After creating your grid you can easily add the centroid lat and long values in decimal degrees using the field calculator. Add two new fields e.g.
centroid-lonandcentroid_lat(making sure to use type Decimal number (real) and set an appropriate precision) and populate them with the following expressions:You should see a result in the attribute table similar to below: