Estimative Indexing is a fertilization strategy developed by Tom Barr. The idea is if you supply everything a plant needs slightly in excess of that need by plants can allow plants to outcompete Algae for nutrients. This is what Tom Barr wrote about it.
It is very popular algae control methode.
It can work very well however sometimes it doesn't. Tom made one mistake in the EI methode. he assumed if your water's general hardness (GH) Value was 3 to 5 or higher you would have enough calcium in the aquarium. However it is now apparent the calcium deficiency is a common problem. GH measures the amount of calcium and magnesium in the water but it doesn't tell you how much calcium or how much magnesium is in the water. It is know known that many people having problems with EI dosing of Calcium or magnesium deficiency. Additionally Tom overlooks the importance of sulfur.
Plants need Calcium, magnesium and sulfur but Tom mainly talks about Nitrogen, phosphate, and potassium. if you don't dose any one of these you are going to have problems and most fertilizers don't have calcium which makes the problem worse. he has stated that if the EI methode is not working the problem is likely CO2 not a fertilizer nutrient which didn't help since we now know many people have issues with calcium, magnesium, and sulfur deficiencies. Additionally there are 15 nutrients that need to be in fertilizers and if any one is deficient your plant growth can slow down.
The best solution is stop focusing on the algae and dose nutrients to get the plants growing properly and maybe check your CO2. The algae already in the tank will be hard to clean off without the help of an SAE but it is not going to matter until you get your plants growing since it will keep growing on your plants.
This is good solid advise but there is another possible cause of the problem. If you get an excess amount of one nutrient in the aquarium the water can sometimes become toxic to plants and slow plant growth. Adding more fertilizer can a sometimes make the toxic excess nutrient problem worse
I had a hair algae problem and I fixed it by switching to EI dosing. When I had the problem I was only dosing excel and flourish just like you. Since I've started adding the other nutrients my plants have out competed the hair algae and I no longer have an issue.
If you have an excess of a nutrient slowing plant growth it can be really had to identify which nutrient it is or to even confirm an excess is present. In that case sometimes changing to a different fertilizer can help. More and larger water changes may also help. There is not golden rule on how much of each nutrient is needed in a fertilizer. some companies might not have enough causing deficiencies while others may have too much. You might want to try a different fertilizer. Many of the EI based fertilizers on the market are a good place to start if you want explore this option.