The hoses are both BH90, but looking at the 105 flat mount specs - it shows SM-BH90-JK-SSR, SM-BH90-J-SSR, but on the SLX mtb caliper specs show SM-BH90-SBM.
Are these different?
Yes.
BH90-SS = straight - straight connections
BH90-SB = straight - banjo connections
https://productinfo.shimano.com/#/co...460&acid=C-186
Deore brakes use a hose with the straight - straight connections like the 105 brakes.
SLX brakes use a hose with straight - banjo connections, you would need new hose for the SLX brake.
https://productinfo.shimano.com/#/spec
https://si.shimano.com/#/