Hello Nancy -
When I see new leaves that look yellow except in the vein of the leaf, that means there is chlorosis.  If it happens in the newer leaves, that usually means the plant is lacking iron, or is lacking the means of using the iron it has. 
I found a link that explains nutritional deficiencies.  https://www.udel.edu/academics/colleges/canr/cooperative-extension/fact-sheets/identifying-nutrient-deficiencies-in-ornamental-plants/
Here is an interesting link:  https://extension.okstate.edu/fact-sheets/identifying-and-correcting-iron-deficiencies-in-ornamentals.html
What I see in this article is that if you are overwatering your plant, it can get chlorosis.  Also, if you use a fertilizer with high phosphorus, you can also get chlorosis.  Do you use well water to water your cactus?  The pH of well water is usually high.  City water is adjusted to 7, fairly neutral.  The article also points out that you need to have soil that isn't compacted.  
I've given you some data to think about.  You can look at your Miracle Grow and see how much phosphorus is in it.  Possibly cut back on watering a bit, if you are keeping your plant wet. 
I hope this helps your Thanksgiving Cactus recover.