Nitrates indirectly contribute to algae. Algae is caused by light, and obviously some nutrients need to be present to feed it. High nitrate levels usually indicate the biological equilibrium is off or not balanced, caused for example by overfeeding, insufficient water changes, no or too few live plants, overcrowding fish. Nitrates should never be allowed to go above 20ppm; although higher is not perhaps dangerous to most fish, it is a fact that in nature fish live in water that is zero or near zero in nitrates, assuming it is healthy and all that. So this tells us that no fish are expecting to be in water with nitrates, therefore low nitrate is probably going to mean better fish health.
Live plants will always reduce nitrates, and in well-planted tanks that are balanced it is common to have nitrates at 0-5 ppm or maybe 10 max if there are more fish. Plants need nitrogen, and they prefer it as ammonium which comes from ammonia, and plants are faster at grabbing it than bacteria, so the bacteria are fewer with live plants (assuming again a balanced fish load) and this means less ammonia is converted by bacteria to nitrite and less nitrite to nitrate. Many plants can assimilate nitrite and nitrate as their nitrogen as well, but studies have shown that it is "more work" because they have to change the nitrite and nitrate back into ammonium; so it is easier for the plants to simply grab the ammonium/ammonia.
Algae of course is a plant; higher plants are better at utilizing nutrients and thus out-compete algae, all else being equal. But if the light is greater than the available nutrients, plants can't use it and will slow or cease photosynthesizing, and algae is quick to take advantage. And of course slower-growing plants use less nutrients and thus less light, so algae is even faster with Anubias. Adding your floating plants will significantly alter this balance in favour of the plants and not algae.
To your second question, my direct answer is short: I don't know, having never used this product or even seen it. However, I would caution you that it is not something i would myself try, at least not without knowing a great deal about it. Excel is also a carbon supplement, and i have frequently recommended against using it. These products set up a new level of balance, and light and the other nutrients must also be increased or the plants cannot gain from the extra carbon. And as we noted earlier in this thread, there is quite a lot of CO2 occurring naturally in all aquaria.