Good news is, you were most likely hit by some filter setting in the firewall, and they probably won't bother checking any further than looking at some summary figures in monthly traffic report.
will my employer see that I accessed this site?
They definitely know what link you have tried to access - anything that shows in your address bar that has https - so if you visited gmail they'd see you stayed at https://www.google.com/intl/en/gmail//# for 5 minutes. They will not have its contents though, as the traffic is encrypted.
They know about any web page and it's contents without encryption (http://) if they sniff packets.
They know the IP and MAC address of your device and credentials you have used to connect.
Depending on the policy of your company just the credentials might point to you.
If the device you have used belongs to your employer, they know it's you.
If the phone you used is private and the authentication method for your Wifi is some global password (something like YourCompanyNameWifi/ some_global_pass) it's less likely to be traced back to you, but still possible (multiple passwords might be generated for the single WiFi network, they might trace it by access point you were connected to as it's location is known etc.)
And there is always metadata, f.e. if your 'that guy' that is fascinated by turtles and what you did earlier was googling some cute turtles on google images, well, they will assume it's you.
Will they know that link was not actually porn?
Not really. They might use global lists or some custom made ones to filter traffic, and they usually target sites as a whole and don't bother with specific links to its resources.
will flag their system or notify them somehow even though the site isnt actually porn?
If they use global lists, this will update automatically without them ever noticing. If it's their own custom filter, then it's probably a good idea to notify them about the mistake.
Don't be too stressed about it, while I am not currently an admin in my company, I work as a programmer and lots of resources and sites I try to access are being hit by our web filter settings (we use Fortinet solutions).
If you need to access some given site and it's blocked, notify the IT, they will know what to do.