That looks to me like one of those proprietary radiator sealing gunk applications... like Radseal or whatever. Y'know, for lazy thick people who CBA to fix a leaky radiator properly? They put a dose in and it doesn't fix the leak so they put in another... and then another... and then the leak does stop largely because the entire system is choked with that crap
When I was blighted with MR2 ownership, it had been abused like that by the previous owner (though he did at least admit it). IIRC it took the best part of two days of pretty much solid flushing through with a hose to clear out the crap. After several hours gouging it out of every orifice I could access with a screwdriver. Honestly, words fail me at the barbarism people inflict on their car engines
EDIT: this is what was in my MR2, Looks similar doesn't it?
