The graph does not look like there is an effect at all as there is no inhibition judging from your y-axis that says % inhibition. Consequently, the fit of this data is inappropriate leading to strange results as you fit a dose-response model into data that do not show a dose-response. Of course the program will fit anything into the data you put in there because it does not know that you would expect a bottom asymptote of 0% and a top asymptote of 100% inhibition.

Anyway, the IC50 is a special case of the EC50 but in the end both are determined in the same way. So the IC50 is an EC50 for inhibitors or antagonists, whereas an EC50 could also be determined for any effector such as an agonist as well, see here: https://en.wikipedia.org/wiki/EC50

As Jonas says: there is no mathematical relationship between the two, they are simply defined differently - and lots of people use the symbols and definitions incorrectly, even when they know better, since the incorrect usage is almost standard.  IC50, for example, is often used in cell growth assays, even though GI50 would be more correct.  EC50 is just the general case for these values: most people use something at least a bit more specific.  It's like how all squares are rectangles, but most people call them squares anyways.

Your graph is probably giving you EC50 because the software neither knows, nor cares, what specifically you are measuring - it just draws a curve, and tells you what concentration gave you half of the maximal effect you observe.